Power converter

ABSTRACT

This power converter comprises a plurality of switching elements, an inductor, and a smoothing capacitor. An input current of low frequency from the AC source is intermitted by the switching element turned on and off at high frequency, and an output current to a load is restricted by the inductor. These switching elements are controlled in different patterns. While these patterns are repeated, a current is always supplied to the load and inductor. Therefore, the power converter can provide a desired output power, making the inductance required for the inductor into the minimum. That is, the power converter can combine miniaturization and high-efficiency.

TECHNICAL FIELD

[0001] The present invention relates to a power converter which providesan electric power for a load from an AC source.

BACKGROUND ART

[0002] Japanese Non-examined Patent Publication No.8-223915 discloses aconventional power converter. This power converter changes AC powerwhich is from an AC source into DC power, and gives the DC power to aload. A boost converter and a back converter are arranged in parallel,and either of them operates according to power-supply voltage. These twoconverters include a switching element, respectively, and supply acurrent to the load, improving harmonic distortion, by making eachswitching element turn on and off at a frequency higher enough than thefrequency of the AC power. This power converter includes two or moreswitching elements and one inductor which is shared by these converters.The switching elements are controlled in different patterns in order togive a plurality of current supplying modes. By repeating these currentsupplying modes, power is supplied to the load, improving the harmonicdistortion. However, because, in one of these modes, the current doesnot flow through the inductor and the load at the same time, energy isstored in the inductor while the current is not supplied to the load.So, the amount of the current inputted into the inductor becomes largetemporarily, compared with the average output current to the load. Forthis reason, the inductor and the switching elements need to withstandvoltage; therefore, it was difficult to miniaturize the circuit.

DISCLOSURE OF THE INVENTION

[0003] In view of the above problem, the object of the present inventionis to provide a power converter which has little harmonic distortion andcan combine miniaturization and high-efficiency.

[0004] The power converter in accordance with the present inventioncomprises a plurality of switching elements which turns on and offrepetitively to interrupt an input current from AC source to provide anoutput current to a load, an inductor provided in a path of the inputcurrent from the AC source to the load, a smoothing capacitor whichsmoothens the input current to the load, a control circuit forcontrolling the switching elements to turn on and off. The inductor andthe load are connected in series across the AC source so that thecurrent from the AC source can flow to the load directly. The inductorand the load are connected in series across the smoothing capacitor,therefore, the current flowing through the smoothing capacitor issupplied to the load, and a low frequency ripple is reduced.

[0005] The control circuit controls the plurality of the switchingelements to turn on and off in different patterns to give a firstcurrent supplying mode and a second current supplying mode. The firstcurrent supplying mode supplies the input current from the AC source ina closed loop including the inductor and the load, during which thecurrent from the AC source is fed directly to the load. The secondcurrent supplying mode supplies the output current to the load in aclosed loop including the inductor and the load but excluding the ACsource, during which energy stored in the inductor supplies a current tothe load. The control circuit repeats the first current supplying modeand the second current supplying mode alternately during each half cycleof the AC current supplied from the AC source, thereby constantlypassing the current to the inductor and the load. Therefore, a currentloop which always contains both the inductor and the load always exists,and the inductor passes only the input current near the average outputcurrent needed for the load. Accordingly, it becomes possible to use asmall inductor which has small inductance and to improve the harmonicdistortion. Therefore, the miniaturization of the whole device can beattained. That is, an efficient and small power converter can beprovided.

[0006] Preferably, the control circuit controls the plurality of theswitching elements in three different patterns to continuously repeat afirst pattern, a second pattern, and a third pattern in this order. Oneof the three patterns defines one of the first current supplying modeand the second current supplying mode; the remaining two patterns definethe other of the first current supplying mode and the second currentsupplying mode. The voltage applied to the inductor decreases inaccordance with a progress from the first pattern to the third pattern.Therefore, it is possible to supply a sufficient current to the loadthrough the inductor, lowering the peak value of the current which flowsto the inductor. Consequently, the loss reduction of a power conversionand the miniaturization of the device can be attained.

[0007] Furthermore, the first pattern allows the smoothing capacitor topass a discharge current through the inductor, the second pattern keepsthe smoothing capacitor free from the current flowing through theinductor, and the third pattern allows the smoothing capacitor to becharged by the current flowing through the inductor. Therefore, theinductor is always supplied with a current, and the change of thecurrent flowing through the inductor draws a generally trapezoid-shapealong the time-axis. Therefore, the peak of the current flowing throughthe inductor is lowered, and the miniaturization of the inductor can beattained.

[0008] In one concrete circuit arrangement realizing the above powerconverter, a rectifier circuit DB which rectifies the AC current fromthe AC source to give a DC voltage is provided in the power converter.The switching elements comprise a first switching element Q1, a secondswitching element Q2, and a third switching element Q3. The firstswitching element Q1, the second switching element Q2, and the thirdswitching element Q3 are connected in series with the inductor L, theload LD, a first diode D1, and the smoothing capacitor C1 across therectifier circuit DB. A second diode D2 is connected across a seriescircuit of the smoothing capacitor C1 and the third switching elementQ3. The second diode D2 is connected in series with a third diode D3across a series circuit of the inductor L, the first diode D1, the loadLD, and the second switching element Q2. And a fourth diode D4 isconnected in series with the smoothing capacitor across the secondswitching element Q2.

[0009] The control circuit can always pass the current to the load andthe inductor, improving the harmonic distortion, namely, a power-factor,by controlling the first, second, and third switching elements Q1-Q3 inthree different patterns.

[0010] A power converter in accordance with other concrete circuitarrangement includes five switching elements Q1-Q5 in addition to arectifier circuit DB which rectifies the AC current from the AC sourceto give a DC voltage. Each of the second switching element Q2 and thefourth switching element Q4 has a bypass allowing a reverse current toflow across each switching element. The first switching element Q1 andthe second switching element Q2 are connected in series with a firstdiode across the rectifier circuit DB, and the first diode D1 isinserted between a high voltage side of the rectifier circuit DB and thefirst switching element Q1, and a cathode of the first diode D1 isconnected to the first switching element Q1. The third switching elementQ3 and the fourth switching element Q4 are connected in series with asecond diode across the rectifier circuit DB, and the second diode D2 isinserted between a high voltage side of the rectifier circuit DB and thethird switching element Q3, and a cathode of the second diode D2 isconnected to the third switching element Q3. The second switchingelement Q2 and the fourth switching element Q4 are connected through acommon third diode D3 to a low voltage side of the rectifier circuit DB.The inductor L is connected in series with the load LD between theconnection point of the first switching element Q1 with the secondswitching element Q2 and the connection point of the third switchingelement Q3 with the fourth switching element Q4. The fifth switchingelement Q5 is connected in series with the first diode D1, the firstswitching element Q1, the inductor L, the load LD, the fourth switchingelement Q4, and the smoothing capacitor C1 across the rectifier circuitDB. Also, the fifth switching element Q5 is connected in series with thesecond diode D2, the third element Q3, the load LD, the inductor L, theswitching element Q2, and the smoothing capacitor C1 across therectifier circuit DB. The fourth diode D4 is connected in series withthe bypass of the second switching element Q2, the inductor L, and theload LD across the smoothing capacitor C1. And the fifth diode D5 isconnected in series with the bypass of the fourth switching element Q4,the load LD, and the inductor L across the smoothing capacitor.

[0011] The control circuit passes the current to the load in onedirection by making the first and fourth switching elements Q1, Q4 turnon and off at the same time and making both the second and thirdswitching elements Q2, Q3 turn off in the meantime, while making thefifth switching element Q5 turn on and off on a predetermined cycle. Andalso, the control circuit passes the current to the load in the reversedirection by making the second and third switching elements Q2, Q3 turnon and off at the same time and making both of the first and fourthswitching elements Q1, Q4 turn off in the meantime, while making thefifth switching element Q5 turn on and off on a predetermined cycle. Bythis, the control circuit can drive the load by the alternating currentof a predetermined frequency. Therefore, a power conversion of AC-DC anda power conversion of DC-AC can be attained at the same time, alwayspassing the current to the load and the inductor, by using only fiveswitching elements. Because each of the second switching element Q2 andthe fourth switching element Q4 has the bypass allowing the reversecurrent to flow across each switching element, the current produced fromthe energy stored at the inductor L charges the smoothing capacitor C1through either the second switching element Q2 or the fourth switchingelement Q4.

[0012] A power converter in accordance with other concrete circuitarrangement includes five switching elements Q1-Q5 in addition to arectifier circuit DB which rectifies the AC current from the AC sourceto give a DC voltage. The first switching element Q1 and the secondswitching element Q2 are connected in series with a first diode D1across the rectifier circuit DB, and the first diode D1 is insertedbetween a high voltage side of the rectifier DB and the first switchingelement Q1, and the first diode D1 has a cathode connected to the firstswitching element Q1. The third switching element Q3 and the fourthswitching element Q4 are connected in series with a second diode D2across the rectifier circuit DB, and the second diode D2 is insertedbetween a high voltage side of the rectifier DB and the third switchingelement Q3, and the second diode D2 has a cathode connected to the thirdswitching element Q3. The inductor L is connected in series with theload LD between the connection point of the first switching element Q1with the second switching element Q2 and the connection point of thethird switching element Q3 with the fourth switching element Q4. Aseries circuit of the first diode D1, the first switching element Q1,the inductor L, the load LD, and the fourth switching element Q4 isconnected in series with the fifth switching element Q5, and also, aseries circuit of the second diode D2, the third switching element Q3,the inductor L, the load LD, and the second switching element Q2 isconnected in series with the fifth switching element Q5. The AC source,the rectifier circuit DB, the first diode D1, the first switchingelement Q1, the inductor L, the load LD, and the third diode D3 areconnected in series across the smoothing capacitor C1. And the ACsource, the rectifier circuit DB, the second diode D2, the thirdswitching element Q3, the inductor L, the load LD, and the fourth diodeD4 are connected in series across the smoothing capacitor C1.

[0013] The control circuit passes the current to the load in onedirection by making the first and fourth switching elements Q1, Q4 turnon and off at the same time and making both of the second and thirdswitching elements Q2, Q3 turn off in the meantime, while making thefifth switching element Q5 turn on and off on a predetermined cycle. Andalso, the control circuit passes the current to the load in the reversedirection by making the second and third switching elements Q2, Q3 turnon and off at the same time and making both the first and fourthswitching elements Q1, Q4 turn off, while making the fifth switchingelement Q5 turn on and off on a predetermined cycle. Therefore, thecontrol circuit can drive the load by the alternating current of apredetermined frequency. And, the power conversion of AC-DC and thepower conversion of DC-AC can be attained at the same time by using onlyfive switching elements, always passing the current to the load and theinductor.

[0014] A power converter in accordance with other concrete circuitarrangement is designed to pass the alternating current to the load byusing four switching elements Q1-Q4. The first switching element Q1 andthe second switching element Q2 are connected in series with theinductor L and the load LD across the AC source, and the first switchingelement Q1 and the third switching element Q3 are connected in serieswith the inductor L and the load LD across the AC source. Each of thesecond switching element Q2 and the third switching element Q3 has abypass allowing a reverse current to flow across each switching element.A series circuit of a first smoothing capacitor C1 and a secondsmoothing capacitor C2 is connected across a series circuit of thesecond switching element Q2 and the third switching element Q3. A seriescircuit of a first diode D1 and a second diode D2 are connected acrossthe series circuit of the second switching element Q2 and the thirdswitching element Q3. A diode bridge D11-D14 is inserted between theconnection point of the first smoothing capacitor C1 with the secondsmoothing capacitor C2 and the AC source, each input terminal of thediode bridge is connected to the connection point of the first smoothingcapacitor C1 with the second smoothing capacitor C2 and the AC source,respectively. The first switching element Q1 is connected between outputterminals of the diode bridge D11-D14. A series circuit of a third diodeD3 and a fourth diode D4 is connected across a series circuit of thefirst diode D1 and the second diode D2. The inductor L and the load LDare connected in series between the connection point of the first diodeD1 with the second diode D2 and the connection point of the third diodeD3 with the fourth diode D4. The fourth switching element Q4 isconnected across the series circuit of the third diode D3 and the fourthdiode D4. A series circuit of a fifth diode D5 and a sixth diode D6 isconnected across the series circuit of the first diode D1 and the seconddiode D2, and the AC source is inserted between the connection point ofthe first diode D1 with the second diode D2 and the connection point ofthe fifth diode D5 with the sixth diode D6.

[0015] The control circuit passes the current to the load in onedirection by making the second switching element Q2 turn on and off andmaking the third switching element Q3 turn off in the meantime, whilemaking the first and fourth switching elements Q1, Q4 turn on and offalternately, and also, the control circuit passes the current to theload in the reverse direction by making the third switching element Q3turn on and off and making the second switching element turn off in themeantime, while making the first and fourth switching elements Q1, Q4turn on and off alternately. Therefore, the control circuit can providethe alternating current of a predetermined frequency for the load bysetting up the time from the on/off control action of one of the secondswitching element Q2 and the third switching element Q3 to the on/offcontrol action of the other of the switching elements Q2 and Q3. So, thepower conversion of AC-DC and the power conversion of DC-AC can beattained at the same time by using only four switching elements, alwayspassing the current to the load and the inductor.

[0016] A power converter in accordance with other concrete circuitarrangement is designed to pass the alternating current to the load byusing four switching elements Q1-Q4. The first switching element Q1 andthe second switching element Q2 are connected in series with theinductor L and the load LD across the AC source, and the first switchingelement Q1 and the third switching element Q3 are connected in serieswith the inductor L and the load LD across said AC source. Each of thesecond switching element Q2 and the third switching element Q3 has abypass allowing a reverse current to flow across each switching element.A series circuit of a first diode D1 and a second diode D2 is connectedacross a series circuit of the second switching element Q2 and the thirdswitching element Q3. A series circuit of a first smoothing capacitor C1and a second smoothing capacitor C2 is connected across the seriescircuit of the second switching element Q2 and the third switchingelement Q3. A diode bridge D11-D14 is inserted between the connectionpoint of the first smoothing capacitor C1 with the second smoothingcapacitor C2 and one terminal of the AC source, and each input terminalof the diode bridge is connected to the connection point of the firstsmoothing capacitor C1 with the second smoothing capacitor C2 and theterminal of the AC source, respectively. The first switching element Q1is connected between output terminals of the diode bridge D11-D14, andthe one terminal of the AC source is connected with the connection pointof the first diode D1 with the second diode D2. A diode bridge D3-D6 isinserted between the connection point of the first diode D1 with thesecond diode D2 and the connection point of the second switching elementQ2 with the third switching element Q3. The diode D3 is connected inseries with said diode D4. The diode D5 is connected in series with thediode D6. The inductor L and the load LD are connected in series betweenthe connection point of the diode D3 with the diode D4 and theconnection point of the diode D5 with the diode D6. The fourth switchingelement Q4 is connected across a series circuit of the fifth diode D5and the sixth diode D6.

[0017] The control circuit passes the current to the load in onedirection by making the second switching element Q2 turn on and off andmaking the third switching element Q3 turn off in the meantime, whilemaking the first and fourth switching element Q1, Q4 turn on and offalternately, and also, the control circuit passes the current to theload in the reverse direction by making the third switching element Q3turn on and off and making the second switching element Q2 turn off inthe meantime, while making the first and fourth switching elements Q1,Q4 turn on and off alternately. Therefore, the control circuit canprovide the alternating current of a predetermined frequency for theload by setting up the time from the on/off control action of one of thesecond switching element Q2 and the third switching element Q3 to theon/off control action of the other of the switching elements Q2, Q3. So,the power conversion of AC-DC and the power conversion of DC-AC can beattained at the same time by using only four switching elements, alwayspassing the current to the load and the inductor.

[0018] A power converter in accordance with other concrete circuitarrangement is designed to pass the alternating current to the load byusing four switching elements Q1-Q4. The first switching element Q1 anda first smoothing capacitor C1 are connected in series with the inductorL and the load LD across the AC source, and the second switching elementQ2 and a second smoothing capacitor C2 are connected in series with theinductor L and the load LD across the AC source. Each of the firstswitching element Q1 and the second switching element Q2 has a bypassallowing a reverse current to flow across each switching element. Thefirst switching element Q1 and the second switching element Q2 areconnected in series, and a series circuit of the first smoothingcapacitor C1 and the second smoothing capacitor C2 is connected acrossthe series circuit of the first switching element Q1 and the secondswitching element Q2. A first diode D1 and the third switching elementQ3 are connected in series across a series circuit of the inductor L andsaid load LD, and a second diode D2 and the fourth switching element Q4are connected in series across the series circuit of the inductor L andthe load LD. A series circuit of the third switching element Q3 and thefourth switching element Q4 is connected across a series circuit of thefirst diode D1 and the second diode D2, and the AC source is insertedbetween the connection point of the first switching element Q1 with thesecond switching element Q2 and the connection point of the first diodeD1 with the second diode D2. The load LD, the inductor L, the AC source,and the bypass of the first switching element Q1 are connected in seriesacross the first smoothing capacitor C1. The bypass of the secondswitching element Q2, the AC source, the inductor L, and the load LD areconnected in series across the second smoothing capacitor C2.

[0019] The control circuit passes the current to the load in onedirection by making the second and third switching elements Q2, Q3 turnoff while making the first and fourth switching elements Q1, Q4 turn onand off alternately. And also, the control circuit passes the current tothe load in the reverse direction by making the first and fourthswitching elements Q1, Q4 turn off while making the second and thirdswitching elements Q2, Q3 turn on and off alternately. Therefore, thecontrol circuit can provide the alternating current of a predeterminedfrequency for the load by setting up the time from the alternatingon/off control action of the first switching element Q1 and the fourthswitching element Q4 to the alternating on/off control action of thesecond switching element Q2 and the third switching element Q3. So, thepower conversion of AC-DC and the power conversion of DC-AC can beattained at the same time by using only four switching elements, alwayspassing the current to the load and the inductor.

[0020] A power converter in accordance with other concrete circuitarrangement is designed to pass the alternating current to the load byusing four switching elements Q1-Q4. The first switching element Q1 anda first diode D1 are connected in series with the inductor L and theload LD across the AC source, the second switching element Q2 and asecond diode D2 are connected in series with the inductor L and the loadLD across the AC source. Each of the first switching element Q1 and thesecond switching element Q2 has a bypass allowing a reverse current toflow across each switching element. The first switching element Q1 andthe second switching element Q2 are connected in series, and a seriescircuit of the first diode D1 and the second diode D2 and a smoothingcapacitor C1 are connected across the series circuit of the first andsecond switching elements Q1, Q2. A series circuit of the thirdswitching element Q3 and the fourth switching element Q4 is connectedacross a series circuit of the switching element Q1 and the secondswitching element Q2. The AC source is inserted between the connectionpoint of the first diode D1 with the second diode D2 and the connectionpoint of the third switching element Q3 with the fourth switchingelement Q4. The inductor L and the load LD are inserted in seriesbetween the connection point of the first switching element Q1 with thesecond switching element Q2 and the connection point of the thirdswitching element Q3 with the fourth switching element Q4. The bypass ofthe second switching element Q2, the load LD, the inductor L, the ACsource, and the first diode D1 are connected in series across thesmoothing capacitor C1, and the second diode D2, the AC source, theinductor L, the load LD, and the bypass of the first switching elementQ1 are connected in series across the smoothing capacitor C1.

[0021] The control circuit passes the current to the load in onedirection by making the second and third switching elements Q2, Q3 turnoff while making the first and fourth switching elements Q1, Q4 turn onand off. And also, the control circuit passes the current to the load inthe reverse direction by making the first and fourth switching elementsQ1, Q4 turn off while making the second and third switching elements Q2,Q3 turn on and off. Therefore, the control circuit can provide thealternating current of a predetermined frequency for the load by settingup the time from the on/off control action of the first and fourthswitching elements Q1, Q4 to the on/off control action of the second andthird switching elements Q2, Q3. So, the power conversion of AC-DC andthe power conversion of DC-AC can be attained at the same time by usingonly four switching elements, always passing the current to the load andthe inductor.

[0022] A power converter in accordance with other concrete circuitarrangement is designed to pass the alternating current to the load byusing four switching elements Q1-Q4. A first diode D1 and the firstswitching element Q1 are connected in series with the inductor L and theload LD across the AC source, and the second switching element Q2 and asecond diode D2 are connected in series with the inductor L and the loadLD across the AC source. The first switching element Q1 and the secondswitching element Q2 are connected in series, and the series circuit ofthe first and second switching elements is connected across a seriescircuit of the first diode D1 and the second diode D2. A first smoothingcapacitor C1 and the third switching element Q3 are connected in seriesacross a series circuit of the inductor L and the load LD. The fourthswitching element Q4 and a second smoothing capacitor C2 are connectedin series across the series circuit of the inductor L and the load LD.Each of said third switching element Q3 and said fourth switchingelement Q4 has a bypass allowing a reverse current to flow across eachswitching element. The first smoothing capacitor C1 and the secondsmoothing capacitor C2 are connected in series, and the series circuitof the first and second smoothing capacitors is connected across aseries circuit of the third switching element Q3 and the fourthswitching element Q4. The AC source is connected between the connectionpoint of the first diode D1 with the second diode D2 and the connectionpoint of the first smoothing capacitor C1 with the second smoothingcapacitor C2. The inductor L, the load LD, and the bypass of the thirdswitching element Q3 are connected in series across the first smoothingcapacitor C1. The bypass of the fourth switching element Q4, the loadLD, and the inductor L are connected in series across the secondsmoothing capacitor C2.

[0023] The control circuit passes the current to the load in onedirection by making the second and fourth switching elements Q2, Q4 turnoff while making the first and third switching elements Q1, Q3 turn onand off alternately. And also, the control circuit passes the current tothe load in the reverse direction by making the first and thirdswitching elements Q1, Q3 turn off while making the second and fourthswitching elements Q2, Q4 turn on and off alternately. Therefore, thecontrol circuit can provide the alternating current of a predeterminedfrequency for the load by setting up the time from the alternatingon/off control action of the first and third switching elements Q1, Q3to the alternating on/off control action of the second and fourthswitching elements Q2, Q4. So, the power conversion of AC-DC and thepower conversion of DC-AC can be attained at the same time by using onlyfour switching elements, always passing the current to the load and theinductor.

[0024] A power converter in accordance with other concrete circuitarrangement is designed to pass the alternating current to the load byusing four switching elements Q1-Q4. A first diode D1 and a firstsmoothing capacitor C1 are connected in series with the inductor L andthe load LD across the AC source, and a second diode D2 and a secondsmoothing capacitor C2 are connected in series with the inductor L andthe load LD across the AC source. The first diode D1 and the seconddiode D2 are connected in series, and the series circuit of the firstand second diodes is connected across a series circuit of the firstsmoothing capacitor C1 and the second smoothing capacitor C2. A seriescircuit of the first switching element Q1 and the second switchingelement Q2 is connected across a series circuit of the first diode D1and the second diode D2. A series circuit of a third diode D3 and thethird switching element Q3 is connected across a series circuit of theinductor L and the load LD, and a series circuit of a fourth diode D4and the fourth switching element Q4 is connected across the seriescircuit of the inductor L and the load LD. A series circuit of the thirdswitching element Q3 and the fourth switching element Q4 is connectedacross a series circuit of the third diode D3 and the fourth diode D4.The AC source is inserted between the connection point of the firstdiode D1 with the second diode D2 and the connection point of the firstswitching element Q1 with the second switching element Q2.

[0025] The control circuit passes the current to the load in onedirection by making the second and third switching elements Q2, Q3 turnoff while making the first and fourth switching elements Q1, Q4 turn onand off alternately. And also, the control circuit passes the current tothe load in the reverse direction by making the first and fourthswitching elements Q1, Q4 turn off while making the second and thirdswitching elements Q2, Q3 turn on and off alternately. Therefore, thecontrol circuit can provide the alternating current of a predeterminedfrequency for the load by setting up the time from the alternatingon/off control action of the first and fourth switching elements Q1, Q4to the alternating on/off control action of the second and thirdswitching elements Q2, Q3. So, the power conversion of AC-DC and thepower conversion of DC-AC can be attained at the same time by using onlyfour switching elements, always passing the current to the load and theinductor.

[0026] A power converter in accordance with other concrete circuitarrangement is designed to pass the alternating current to the load byusing four switching elements Q1-Q4. A first diode D1, the firstswitching element Q1, and a second diode D2 are connected in series withthe inductor L and the load LD across the AC source, and the first diodeD1, the second switching element Q2, the second diode D2, and asmoothing capacitor C1 are connected in series with the inductor L andthe load LD across the AC source. And also, a third diode D3, the thirdswitching element Q3, and a fourth diode D4 are connected in series withthe inductor L and the load LD across the AC source, and the third diodeD3, the fourth switching element Q4, the smoothing capacitor C1, and thefourth diode D4 are connected in series with the inductor L and the loadLD across the AC source. Each of the second switching element Q2 and thefourth switching element Q4 has a bypass allowing a reverse current toflow across each switching element. A series circuit of the firstswitching element Q1, the fourth switching element Q4, the inductor L,and the load LD is connected across the smoothing capacitor C1, andalso, a series circuit of the second switching element Q2, the thirdswitching element Q3, the inductor L, and the load LD is connectedacross the smoothing capacitor C1. The second diode D2, the AC source,the first diode D1, the load LD, the inductor L, and the bypass of thesecond switching element Q2 are connected in series across the smoothingcapacitor C1. And the fourth diode D4, the AC source, the third diodeD3, the inductor L, the load LD, and the bypass of the fourth switchingelement Q4 are connected in series across the smoothing capacitor C1.

[0027] The control circuit passes the current to the load in onedirection by making the second and third switching elements Q2, Q3 turnoff while making the first and fourth switching elements Q1, Q4 turn onand off with different duty ratio. And also, the control circuit passesthe current to the load in the reverse direction by making the first andfourth switching elements Q1, Q4 turn off while making the second andthird switching elements Q2, Q3 turn on and off with different dutyratio. Therefore, the control circuit can provide the alternatingcurrent of a predetermined frequency for the load by setting up the timefrom the on/off control action of the first and fourth switchingelements Q1, Q4 to the on/off control action of the second and thirdswitching elements Q2, Q3. So, the power conversion of AC-DC and thepower conversion of DC-AC can be attained at the same time by using onlyfour switching elements, always passing the current to the load and theinductor.

[0028] It is preferable that the inductor L is connected with thesmoothing capacitor through a rectifying device in a power converter inaccordance with other concrete circuit arrangement. Also, it ispreferred that the inductor L has a primary winding n1 and a secondarywinding n2, and a current is fed to the load through the primary windingn1, and the secondary winding n2 is connected with the smoothingcapacitor C1 through the rectifying device, and the smoothing capacitorC1 is charged by the current generated in the secondary winding. Byusing the inductor including the primary winding n1 and the secondarywinding n2 as mentioned above, it is possible to set the voltage of thesmoothing capacitor to a desired value by selecting the turn ratio ofthe primary winding n1 and the secondary winding n2 appropriately,whereby, the freedom of a circuit design can be improved.

[0029] Furthermore, a power converter in accordance with other concretecircuit arrangement converts power using a rectifier circuit DB whichrectifies the AC current from said AC source and two switching elementsQ1 and Q2. The first switching element Q1 is connected in series withthe inductor L and the load LD across the rectifier circuit DB, and afirst diode D1, the smoothing capacitor C1, and a second diode D2 areconnected in series across the inductor L. The first diode D1 and thesecond diode D2 define the above rectifying device. A series circuit ofthe second switching element Q2, the inductor L, and the load LD isconnected across the smoothing capacitor C1.

[0030] The control circuit can improve a power-factor and can limit thecurrent to the load, always passing the current to the load and theinductor, by using two switching elements Q1, Q2 which are controlled sothat they can have both a period in which they are turned on and offalternately and a period in which they are turned off at the same time.

[0031] Also, it is prevented that the supply voltage to the load willbecome superfluous, because the first diode D1 is connected in serieswith the inductor L and the smoothing capacitor C1 across the rectifiercircuit DB and the current is shunted to the smoothing capacitor throughthe first diode D1 near the peak voltage of the AC source.

[0032] Furthermore, a power converter in accordance with other concretecircuit arrangement converts power using a rectifier circuit DB whichrectifies the AC current from said AC source, two switching elements Q1and Q2, and an inductor having a primary winding n1 and a secondarywinding n2. The first switching element Q1 is connected in series withthe primary winding n1 of the inductor L and the load LD across therectifier circuit DB. The second switching element Q2, the inductor L,and the load LD are connected in series across the smoothing capacitorC1. The secondary winding n2 and a first diode D1 are connected acrossthe smoothing capacitor C1, and the first diode D1 defines therectifying device. The load LD and a second diode D2 are connected inseries across the primary winding n1.

[0033] So, the control circuit can improve a power-factor and can limitthe current to the load, always passing the current to the load and theinductor, by using two switching elements which are controlled so thatthey can have both a period in which they are turned on and offalternately and a period in which they are turned off at the same time.Furthermore, the freedom of a circuit design can be raised, because itis possible to set the voltage of the smoothing capacitor to a desiredvalue by selecting the turn ratio of the primary winding n1 and thesecondary winding n2 suitably.

[0034] Furthermore, a power converter in accordance with other concretecircuit arrangement converts power using a rectifier circuit DB whichrectifies the AC current from said AC source and two switching elementsQ1 and Q2. In this circuit, the first switching element Q1 is connectedin series with the inductor L, the load LD, the smoothing capacitor C1,and the second switching element Q2 across the rectifier circuit DB. Andthe first switching element Q1 is connected in series with the inductorL, the load LD, and a first diode D1 across the rectifier circuit DB. Aseries circuit of a second diode D2, the smoothing capacitor C1, and athird diode D3 is inserted across the inductor L, and the second diodeD2 and the third diode D3 define the above rectifying device whichpasses the current to the smoothing capacitor from the inductor.

[0035] The control circuit can improve a power-factor and can limit thecurrent to the load, always passing the current to the load and theinductor, by using two switching elements which are controlled so thatthey can have a period in which both of them are turned on, a period inwhich either of them is turned off, and a period in which both of themare turned off. Further, the control circuit can provide a stable powerfor the load, because the surplus power flowing through the inductor isshunted to the smoothing capacitor through the second and third diodesD2 and D3. And, there are few idle periods of the input current, and thesuppression effect on the harmonic component is high, because the firstand second switching elements pass the input current from the AC sourceto both the inductor and the load in two patterns out of the threedifferent patterns.

[0036] Furthermore, a power converter in accordance with other concretecircuit arrangement is designed to pass the alternating current to theload by using four switching elements Q1-Q4. Each of the first switchingelement Q1 and the second switching element Q2 has a bypass allowing areverse current to flow across each switching element. In this circuit,a first diode D1, the first switching element Q1, the inductor L, andthe load LD are connected in series across the AC source. And the loadLD, the inductor L, the second switching element Q2, and a second diodeD2 are connected in series across the AC source. The third switchingelement Q3 and the bypass of the second switching element Q2 areconnected in series across a series circuit of the inductor L and theload LD. And, a third diode D3, a smoothing capacitor C1, and the bypassof the second switching element Q2 are connected in series across theinductor L. The bypass of the first switching element Q1 and the fourthswitching element Q4 are connected in series across the series circuitof the inductor L and the load LD. The bypass of the first switchingelement Q1, the smoothing capacitor C1, and a fourth diode D4 areconnected in series across the inductor L. And, the third diode D3 andthe fourth diode D4 define the above rectifying device which passes acurrent from the inductor L to the smoothing capacitor.

[0037] The control circuit passes the current to the load in onedirection by controlling the first and third switching elements Q1 andQ3 so that both switching elements will repeat three patterns comprisinga period in which both switching elements are turned on at the sametime, a period in which either of them is turned on, and a period inwhich both of them are turned off, while making the second and fourthswitching element Q2 and Q4 turn off. And the control circuit passes thecurrent to the load in the reverse direction by controlling the secondand fourth switching element Q2 and Q4 so that both switching elementswill repeat three patterns comprising a period in which both switchingelements are turned on at the same time, a period in which either ofthem is turned on, and a period in which both of them are turned off,while making the first and third switching elements Q1 and Q3 turn off.Therefore, the control circuit can provide the alternating current of apredetermined frequency for the load by setting up the time from theon/off control action of the first and third switching elements Q1, Q3to the on/off control action of the second and fourth switching elementsQ2, Q4. So, the power conversion of AC-DC and the power conversion ofDC-AC can be attained at the same time by using only four switchingelements, always passing the current to the load and the inductor.

[0038] Further, the control circuit can provide a stable power to theload, because the surplus power flowing through the inductor is shuntedto the smoothing capacitor through the third and fourth diodes D3 andD4.

[0039] Furthermore, a power converter in accordance with other concretecircuit arrangement is designed to pass the alternating current to theload by using four switching elements Q1-Q4. Each of the first switchingelement Q1 and the second switching element Q2 has a bypass allowing areverse current to flow across each switching element. A first diode D1,the first switching element Q1, the primary winding n1 of the inductorL, and the load LD are connected in series across the AC source, andalso, the load LD, the primary winding n1, the second switching elementQ2, and a second diode D2 are connected in series across the AC source.The third switching element Q3 and the bypass of the second switchingelement Q2 are connected in series across a series circuit of theprimary winding n1 and the load LD. The bypass of the first switchingelement Q1 and the fourth switching element Q4 are connected in seriesacross the series circuit of the primary winding n1 and the load LD. Aseries circuit of a third diode D3, the smoothing capacitor C1, and afourth diode D4 is connected across the secondary winding n2. A seriescircuit of a fifth diode D5, the smoothing capacitor C1, and a sixthdiode D6 is also connected across the secondary winding n2. A seriescircuit of the first switching element Q1, the primary winding n1, theload LD, the third switching element Q3 is inserted across the smoothingcapacitor C1. A series circuit of the fourth switching element Q4, theload LD, the primary winding n1, the second switching element Q2 is alsoinserted across the smoothing capacitor C1. The third diode D3, thefourth diode D4, the fifth diode D5, and the sixth diode D6 define theabove rectifying device which passes a current from the inductor L tothe smoothing capacitor.

[0040] The control circuit passes the current to the load in onedirection by controlling the first and third switching elements Q1 andQ3 so that both switching elements will repeat three patterns comprisinga period in which both switching elements are turned on at the sametime, and periods in which either of them is turned on, while making thesecond and fourth switching element Q2 and Q4 turn off. And the controlcircuit passes the current to the load in the reverse direction bycontrolling the second and fourth switching elements Q2 and Q4 so thatboth switching elements will repeat three patterns comprising a periodin which both switching elements are turned on at the same time, andperiods in which either of them is turned on, while making the first andthird switching elements Q1 and Q3 turn off. Therefore, the controlcircuit can provide the alternating current of a predetermined frequencyfor the load by setting up the time from the on/off control action ofthe first and third switching elements Q1, Q3 to the on/off controlaction of the second and fourth switching elements Q2, Q4. So, the powerconversion of AC-DC and the power conversion of DC-AC can be attained atthe same time by using only four switching elements, always passing thecurrent to the load and the inductor.

[0041] In this case, it is possible to set the voltage of the smoothingcapacitor to a desired value by selecting the turn ratio of the primarywinding n1 and the secondary winding n2 appropriately, whereby, thefreedom of a circuit design can be raised.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a circuit diagram of a power converter in accordancewith a first embodiment of the present invention.

[0043]FIG. 2 is a time chart showing a control action of switchingelements used in the above circuit.

[0044]FIGS. 3A, 3B, and 3C are explanatory diagrams showing a pass of acurrent flowing through the above circuit when the switching elementsare controlled in different on/off patterns.

[0045]FIGS. 4A, 4B, and 4C are schematic diagrams showing an equivalentcircuit corresponding to FIGS. 3A, 3B, and 3C, respectively.

[0046]FIG. 5 is a graph showing the current flowing through the inductorin the above circuit.

[0047]FIG. 6 is a time chart showing a control action of switchingelements used in a power converter in accordance with a secondembodiment of the present invention.

[0048]FIGS. 7A, 7B, and 7C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0049]FIGS. 8A, 8B, and 8C are schematic diagrams showing an equivalentcircuit corresponding to FIGS. 7A, 7B, and 7C, respectively.

[0050]FIG. 9 is a graph showing the current flowing through the inductorin the above circuit.

[0051]FIG. 10 is a time chart showing a control action of switchingelements used in a power converter in accordance with a third embodimentof the present invention.

[0052]FIGS. 11A, 11B, and 11C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0053]FIGS. 12A, 12B, and 12C are schematic diagrams showing anequivalent circuit corresponding to FIGS. 11A, 11B, and 11C,respectively.

[0054]FIG. 13 is a graph showing the current flowing through theinductor in the above circuit.

[0055]FIG. 14 is a time chart showing a control action of switchingelements used in a power converter in accordance with a fourthembodiment of the present invention.

[0056]FIGS. 15A, 15B, and 15C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0057]FIGS. 16A, 16B, and 16C are schematic diagrams showing anequivalent circuit corresponding to FIGS. 15A, 15B, and 15C,respectively.

[0058]FIG. 17 is a graph showing the current flowing through theinductor in the above circuit.

[0059]FIG. 18 is a time chart showing a control action of switchingelements used in a power converter in accordance with a fifth embodimentof the present invention.

[0060]FIGS. 19A, 19B, and 19C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0061]FIGS. 20A, 20B, and 20C are schematic diagrams showing anequivalent circuit corresponding to FIGS. 19A, 19B, and 19C,respectively.

[0062]FIG. 21 is a graph showing the current flowing through theinductor in the above circuit.

[0063]FIG. 22 is a time chart showing a control action of switchingelements used in a power converter in accordance with a sixth embodimentof the present invention.

[0064]FIGS. 23A, 23B, and 23C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0065]FIGS. 24A, 24B, and 24C are schematic diagrams showing anequivalent circuit corresponding to FIGS. 23A, 23B, and 23C,respectively.

[0066]FIG. 25 is a graph showing the current flowing through theinductor in the above circuit.

[0067]FIG. 26 is a circuit diagram of a power converter in accordancewith a seventh embodiment of the present invention.

[0068]FIG. 27 is a time chart showing a control action of switchingelements used in the above circuit.

[0069]FIGS. 28A, 28B, and 28C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0070]FIG. 29 is a time chart showing a control action of switchingelements used in a power converter in accordance with a eighthembodiment of the present invention.

[0071]FIGS. 30A, 30B, and 30C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0072]FIG. 31 is a time chart showing a control action of switchingelements used in a power converter in accordance with a ninth embodimentof the present invention.

[0073]FIGS. 32A, 32B, and 32C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0074]FIG. 33 is a circuit diagram of a power converter in accordancewith a tenth embodiment of the present invention.

[0075]FIG. 34 is a time chart showing a control action of switchingelements used in the above circuit.

[0076]FIGS. 35A, 35B, and 35C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0077]FIG. 36 is a time chart showing a control action of switchingelements used in a power converter in accordance with a eleventhembodiment of the present invention.

[0078]FIGS. 37A, 37B, and 37C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0079]FIG. 38 is a time chart showing a control action of switchingelements used in a power converter in accordance with a twelfthembodiment of the present invention.

[0080]FIGS. 39A, 39B, and 39C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0081]FIG. 40 is a circuit diagram of a power converter in accordancewith a thirteenth embodiment of the present invention.

[0082]FIG. 41 is a time chart showing a control action of switchingelements used in the above circuit.

[0083]FIGS. 42A, 42B, and 42C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the positivehalf cycle of an AC source.

[0084]FIGS. 43A, 43B, and 43C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the negativehalf cycle of the AC source.

[0085]FIG. 44 is a circuit diagram of a power converter in accordancewith a fourteenth embodiment of the present invention.

[0086]FIG. 45 is a time chart showing a control action of switchingelements used in the above circuit.

[0087]FIGS. 46A, 46B, and 46C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the positivehalf cycle of the AC source.

[0088]FIGS. 47A, 47B, and 47C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the negativehalf cycle of the AC source.

[0089]FIG. 48 is a circuit diagram of a power converter in accordancewith a fifteenth embodiment of the present invention.

[0090]FIG. 49 is a time chart showing a control action of switchingelements used in the above circuit.

[0091]FIGS. 50A, 50B, and 50C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the positivehalf cycle of the AC source.

[0092]FIGS. 51A, 51B, and 51C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the negativehalf cycle of the AC source.

[0093]FIG. 52 is a circuit diagram of a power converter in accordancewith a sixteenth embodiment of the present invention.

[0094]FIG. 53 is a time chart showing a control action of switchingelements used in the above circuit.

[0095]FIGS. 54A, 54B, and 54C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the positivehalf cycle of the AC source.

[0096]FIGS. 55A, 55B, and 55C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the negativehalf cycle of the AC source.

[0097]FIG. 56 is a circuit diagram of a power converter in accordancewith a seventeenth embodiment of the present invention.

[0098]FIG. 57 is a time chart showing a control action of switchingelements used in the above circuit.

[0099]FIGS. 58A, 58B, and 58C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the positivehalf cycle of the AC source.

[0100]FIGS. 59A, 59B, and 59C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the negativehalf cycle of the AC source.

[0101]FIG. 60 is a circuit diagram of a power converter in accordancewith an eighteenth embodiment of the present invention.

[0102]FIG. 61 is a time chart showing a control action of switchingelements used in the above circuit.

[0103]FIGS. 62A, 62B, and 62C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the positivehalf cycle of the AC source.

[0104]FIGS. 63A, 63B, and 63C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the negativehalf cycle of the AC source.

[0105]FIG. 64 is a circuit diagram of a power converter in accordancewith a nineteenth embodiment of the present invention.

[0106]FIG. 65 is a time chart showing a control action of switchingelements used in the above circuit.

[0107]FIGS. 66A, 66B, and 66C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the positivehalf cycle of the AC source.

[0108]FIGS. 67A, 67B, and 67C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the negativehalf cycle of the AC source.

[0109]FIG. 68 is a time chart showing a control action of switchingelements used in a power converter in accordance with a twentiethembodiment of the present invention.

[0110]FIGS. 69A, 69B, and 69C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the positivehalf cycle of the AC source.

[0111]FIGS. 70A, 70B, and 70C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the negativehalf cycle of the AC source.

[0112]FIG. 71 is a circuit diagram of a power converter in accordancewith a twenty-first embodiment of the present invention.

[0113]FIG. 72 is a time chart showing a control action of switchingelements used in the above circuit.

[0114]FIGS. 73A, 73B, and 73C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0115]FIG. 74 is a circuit diagram of a power converter in accordancewith a twenty-second embodiment of the present invention.

[0116]FIGS. 75A, 75B, and 75C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0117]FIG. 76 is a circuit diagram of a power converter in accordancewith a twenty-third embodiment of the present invention.

[0118]FIG. 77 is a time chart showing a control action of switchingelements used in the above circuit.

[0119]FIGS. 78A, 78B, and 78C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns.

[0120]FIG. 79 is a circuit diagram of a power converter in accordancewith a twenty-fourth embodiment of the present invention.

[0121]FIG. 80 is a time chart showing a control action of switchingelements used in the above circuit.

[0122]FIGS. 81A, 81B, and 81C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the positivehalf cycle of the AC source.

[0123]FIGS. 82A, 82B, and 82C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the negativehalf cycle of the AC source.

[0124]FIG. 83 is a circuit diagram of a power converter in accordancewith a twenty-fifth embodiment of the present invention.

[0125]FIGS. 84A, 84B, and 84C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the positivehalf cycle of the AC source.

[0126]FIGS. 85A, 85B, and 85C are explanatory diagrams showing a pass ofthe current flowing through the above circuit when the switchingelements are controlled in different on/off patterns over the negativehalf cycle of the AC source.

BEST MODE FOR CARRYING OUT THE INVENTION

[0127] (First Embodiment)

[0128] A power converter in accordance with a first embodiment of thepresent invention will be explained based on FIGS. 1-5. This powerconverter is designed to convert AC power which is supplied from an ACsource into DC power and to drive a load such as a discharge lamp. Thispower converter includes a rectifier circuit DB which rectifies the ACcurrent from the AC source to give a DC voltage, switching elementswhich comprises a first switching element Q1, a second switching elementQ2, and a third switching element Q3, and a control circuit 1 whichcontrols the switching elements to turn on and off. The first switchingelement Q1, the second switching element Q2, and the third switchingelement Q3 are connected in series with an inductor L, a load LD, afirst diode D1, and a smoothing capacitor C1 across the rectifiercircuit DB. A second diode D2 is connected across a series circuit ofthe smoothing capacitor C1 and the third switching element Q3. Thesecond diode D2 is connected in series with a third diode D3 across aseries circuit of the inductor L, the first diode D1, the load LD, andthe second switching element Q2. And a fourth diode D4 is connected inseries with the smoothing capacitor C1 across the second switchingelement Q2. And, a low pass filter is provided between the AC source andthe rectifier circuit DB.

[0129] The control circuit 1 can always pass the current to both theload and the inductor, improving the harmonic distortion, namely, apower-factor, by controlling the first, second, and third switchingelements Q1-Q3 in three different patterns. The control circuit repeatsthe three different patterns two or more times in the half cycle of theAC current from the AC source, as shown in FIG. 2. In a first pattern,all the switching elements Q1-Q3 are turned on. In a second pattern,only the switching elements Q1 and Q2 are turned on. In a third pattern,all the switching elements Q1-Q3 are turned off. Each switching elementis turned on and off at a frequency higher enough than the frequency ofthe AC source (50-60 Hz), for example, dozens-several hundred kHz.

[0130]FIGS. 3A-3C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 3A, a current I1 flows increasingly througha closed loop; from the AC source to the AC source via the rectifiercircuit DB, the first switching element Q1, the inductor L, the firstdiode D1, the load LD, the second switching element Q2, the smoothingcapacitor C1, the third switching element Q3, and the rectifier circuitDB. The current I1 is accompanied by a discharging current from thesmoothing capacitor C1. A simplified equivalent circuit in this periodis shown in FIG. 4A, which is a series circuit of the inductor L and theload LD connected across a series circuit of the charged smoothingcapacitor C1 and a DC source E which outputs an input voltage Vin.

[0131] In the second period T2, as shown in FIG. 3B, a current I2 flowsincreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first switching element Q1, theinductor L, the first diode D1, the load LD, the second switchingelement Q2, the second diode D2, and the rectifier circuit DB. Thecurrent I2 flows independently of a charge and a discharge of thesmoothing capacitor C1. A simplified equivalent circuit in this periodis shown in FIG. 4B, which is a series circuit of the inductor L and theload LD connected across the DC source E.

[0132] In the third period T3, as shown in FIG. 3C, a current I3 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the first diodeD1, the load LD, the fourth diode D4, the smoothing capacitor C1, thesecond diode D2, and the third diode D3. The current I3 is accompaniedby a charging current to the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 4C, which is a seriescircuit of the inductor L, the load LD, and the smoothing capacitor C1.

[0133] In the first and second period T1 and T2, a first currentsupplying mode is given, in which the current flows through a loopincluding the AC source, the inductor L, and the load LD. In the thirdperiod T3, a second current supplying mode is given, in which thecurrent flows through a loop including the inductor L and the load LDbut excluding the AC source. The control circuit can improve a harmonicdistortion (a power-factor) and limit the current to the load, alwayspassing the current to the load and the inductor, by repeating the firstcurrent supplying mode and the second current supplying modealternately.

[0134] As shown in FIG. 5, the inductor L sees a current IL1 oftrapezoidal waveform over the periods T1 to T3. The periods in which theinductor L contributes to the input current from the AC source are theperiods T1 and T2, and the periods in which the inductor L contributesto the output current to the load LD are the periods T1-T3.

[0135] If the input voltage (the output voltage of the rectifier circuitDB) is Vin, the voltage across the smoothing capacitor C1 is Vc1, andthe voltage across the load LD is VLd, then a voltage VLt1 across theinductor L in the first period T1 is expressed as; VLt1=Vin+Vc1−VLd, anda voltage VLt2 across the inductor L in the second period T2 isexpressed as; VLt2=Lin−VLd, and a voltage VLt3 across the inductor L inthe third period T3 is expressed as; VLt3=−VLd−Vc1. Because Vc1>Vin isalways held in this circuit arrangement, VLt1>VLt2>VLt3 are always held.These voltages VLt1, VLt2, and VLt3 across the inductor L define agradient of the inductor current IL1 in the periods T1 to T3. Thus, agradient in the period T1>a gradient in the period T2>a gradient in theperiod T3, therefore, the inductor current IL1 becomes a trapezoidalwaveform.

[0136] The amount of the power conversion in a desired input and outputvoltage is fluctuated in proportion to the average of the amount of thecurrent flowing through the inductor L. In this embodiment, the inductorcurrent IL which lowers the peak value can be realized by making theinductor current IL into the generally trapezoid-shape as mentionedabove, whereby, the miniaturization of the inductor L can be attained.In addition, since one inductor L contributes to both improving theharmonic distortion and limiting the current, the miniaturization of thepower converter can be attained.

[0137] Furthermore, in this embodiment, because a loop in which theinput current from the AC source flows through the load LD directlythrough the inductor L is formed in the first and second periods T1 andT2, it is possible to reduce the number of the elements through whichthe current from the AC source to the load flows, therefore, the loss ofthe power conversion can be reduced, and the miniaturization of thedevice can be attained.

[0138] Furthermore, in this embodiment, since the output current is fedto the load through the inductor L over all the periods, that is, theinductor L always supplies the current to the load, it is possible tolower the peak value of the inductor current IL. Therefore, the loss ofthe power conversion at the switching elements Q1-Q3, the inductor L,and the diode D1-D4 can be reduced, and the miniaturization of thedevice becomes possible.

[0139] Furthermore, because a current loop including the inductor L andthe load LD is always formed and a current loop including the AC source,the inductor L, and the load LD is formed in the periods T1 and T2, thelength (T1, T2) in which the inductor current IL1 contributes to theinput is shorter than the length (T1, T2, and T3) in which the inductorcurrent contributes to the output to the load LD. If the loss of thecircuit is ignored, the input power and the output power in the powerconverter are equivalent, and when the supply voltage is higher than theload voltage, the output current (a current from the AC source or thesmoothing capacitor to the load) becomes larger than the input current(a current from the AC source to the load and/or the smoothingcapacitor). If the magnitude relation between the input current and theoutput current matches the magnitude relation between the length inwhich the inductor current contributes to the input and the length inwhich the inductor current contributes to the output, it is possible tochange larger current over longer length and suppress the peak currentof choke current. Therefore, this power converter is suitable when thesupply voltage is higher than the load voltage.

[0140] (Second Embodiment)

[0141] A power converter in accordance with a second embodiment of thepresent invention will be explained based on FIGS. 6-9. A circuitarrangement of the power converter is identical to the first embodiment,and a control system of the control circuit 1 is different from thefirst embodiment. The similar parts of these embodiments are identifiedby the same reference character. The control circuit 1 controls thefirst, second, and third switching elements Q1-Q3 in three differenton/off patterns, as shown in FIG. 6. In a first pattern, all theswitching elements Q1-Q3 are turned on. In a second pattern, only thesecond switching element Q2 is turned on. In a third pattern, only thefirst switching elements Q1 is turned on. Each switching element isturned on and off at a frequency higher enough than the frequency of theAC source (50-60 Hz), for example, dozens-several hundred kHz.

[0142]FIGS. 7A-7C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 7A, a current I1 flows increasingly througha closed loop; from the AC source to the AC source via the rectifiercircuit DB, the first switching element Q1, the inductor L, the firstdiode D1, the load LD, the second switching element Q2, the smoothingcapacitor C1, the third switching element Q3, and the rectifier circuitDB. The current I1 is accompanied by a discharging current from thesmoothing capacitor C1. A simplified equivalent circuit in the firstperiod is shown in FIG. 8A, which is a series circuit of the inductor Land the load LD connected across a series circuit of the chargedsmoothing capacitor C1 and a DC source E which outputs an input voltageVin.

[0143] In the second period T2, as shown in FIG. 7B, a current I2 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the first diodeD1, the load LD, the second switching element Q2, the second diode D2,and the third diode D3. The current I2 flows independently of a chargeand a discharge of the smoothing capacitor C1. A simplified equivalentcircuit in this period is shown in FIG. 8B, which is a series circuit ofthe inductor L and the load LD.

[0144] In the third period T3, as shown in FIG. 7C, a current I3 flowsdecreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first switching element Q1, theinductor L, the first diode D1, the load LD, the fourth diode D4, thesmoothing capacitor C1, the second diode D2, and the rectifier circuitDB. The current I3 is accompanied by a charging current to the smoothingcapacitor C1. A simplified equivalent circuit in this period is shown inFIG. 8C, which is a series circuit of the inductor L, the load LD, andthe smoothing capacitor C1 connected across the DC source E.

[0145] In the first and third period T1 and T3, a first currentsupplying mode is given, in which the current flows through a closedloop including the AC source, the inductor L, and the load LD. In thesecond period T2, a second current supplying mode is given, in which thecurrent flows through a closed loop including the inductor L and theload LD but excluding the AC source. That is, the control circuit canimprove a harmonic distortion (a power-factor) and limit the current tothe load, always passing the current to the load and the inductor L, byrepeating the first current supplying mode and the second currentsupplying mode alternately.

[0146] As shown in FIG. 9, the inductor L sees a current IL1 oftrapezoidal waveform over the periods T1 to T3. The periods in which theinductor L contributes to the input current from the AC source are theperiods T1 and T2, and the periods in which the inductor L contributesto the output current to the load LD are the periods T1-T3.

[0147] If the output voltage of the rectifier circuit DB is Vin, thevoltage across the smoothing capacitor C1 is Vc1, and the voltage acrossthe load LD is VLd, then a voltage VLt1 across the inductor L in thefirst period T1 is expressed as; VLt1=Vin+Vc1−VLd, and a voltage VLt2across the inductor L in the second period T2 is expressed as;VLt2=−VLd, and a voltage VLt3 across the inductor L in the third periodT3 is expressed as; VLt3=Vin−Vc1−VLd. Because Vc1>Vin is always held inthis circuit arrangement, VLt1>VLt2>VLt3 are always held. These voltagesVLt1, VLt2, and VLt3 across the inductor L define a gradient of theinductor current IL1 in the periods T1-T3. Thus, a gradient in theperiod T1>a gradient in the period T2>a gradient in the period T3,therefore, the inductor current IL1 becomes a trapezoidal waveform.

[0148] (Third Embodiment)

[0149] A power converter in accordance with a third embodiment of thepresent invention will be explained based on FIGS. 10-13. A circuitarrangement of the power converter is identical to the first embodiment,and a control system of the control circuit 1 is different from thefirst embodiment. The similar parts of these embodiments are identifiedby the same reference character. The control circuit 1 controls thefirst, second, and third switching elements Q1-Q3 in three differenton/off patterns, as shown in FIG. 10. In a first pattern, the second andthird switching elements Q2 and Q3 are turned on. In a second pattern,the first and second switching elements Q1 and Q2 are turned on. In athird pattern, only the first switching element Q1 is turned on. Eachswitching element is turned on and off at a frequency higher enough thanthe frequency of the AC source (50-60 Hz), for example, dozens-severalhundred kHz.

[0150]FIGS. 11A-11C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 11A, a current I1 flows increasingly througha closed loop; from the smoothing capacitor C1 to the smoothingcapacitor C1 via the third switching element Q3, the third diode D3, theinductor L, the first diode D1, the load LD, and the second switchingelement Q2. The current I1 is accompanied by a discharging current fromthe smoothing capacitor C1. A simplified equivalent circuit in the firstperiod is shown in FIG. 12A, which is a series circuit of the inductor Land the load LD connected across the charged smoothing capacitor C1.

[0151] In the second period T2, as shown in FIG. 11B, a current I2 flowsincreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first switching element Q1, theinductor L, the first diode D1, the load LD, the second switchingelement Q2, the second diode D2, and the rectifier circuit DB. Thecurrent I2 flows independently of a charge and a discharge of thesmoothing capacitor C1. A simplified equivalent circuit in this periodis shown in FIG. 12B, which is a series circuit of the inductor L andthe load LD connected across the DC source E which outputs an inputvoltage Vin.

[0152] In the third period T3, as shown in FIG. 11C, a current I3 flowsdecreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first switching element Q1, theinductor L, the first diode D1, the load LD, the fourth diode D4, thesmoothing capacitor C1, the second diode D2, and the rectifier circuitDB. The current I3 is accompanied by a charging current to the smoothingcapacitor C1. A simplified equivalent circuit in this period is shown inFIG. 12C, which is a series circuit of the inductor L, the load LD, andthe smoothing capacitor C1 connected across the DC source E.

[0153] In the second and third period T2 and T3, a first currentsupplying mode is given, in which the current flows through a closedloop including the AC source, the inductor L, and the load LD. In thefirst period T1, a second current supplying mode is given, in which thecurrent flows through a closed loop including the inductor L and theload LD but excluding the AC source. That is, the control circuit canimprove a harmonic distortion (a power-factor) and limit the current tothe load, always passing the current to the load and the inductor L, byrepeating the first current supplying mode and the second currentsupplying mode alternately.

[0154] As shown in FIG. 13, the inductor L sees a current IL1 oftrapezoidal waveform over the periods T1 to T3. The periods in which theinductor L contributes to the input current from the AC source are theperiods T2 and T3, and the periods in which the inductor L contributesto the output current to the load LD are the periods T1-T3.

[0155] If the output voltage of the rectifier circuit DB is Vin, thevoltage across the smoothing capacitor C1 is Vc1, and the voltage acrossthe load LD is VLd, then a voltage VLt1 across the inductor L in thefirst period T1 is expressed as; VLt1=Vc1−VLd, and a voltage VLt2 acrossthe inductor L in the second period T2 is expressed as; VLt2=Vin−VLd,and a voltage VLt3 across the inductor L in the third period T3 isexpressed as; VLt3=Vin−Vc1−VLd. Because Vc1>Vin is always held in thiscircuit arrangement, VLt1>VLt2>VLt3 are always held. These voltagesVLt1, VLt2, and VLt3 across the inductor L define a gradient of theinductor current IL1 in the periods T1-T3. Thus, a gradient in theperiod T1>a gradient in the period T2>a gradient in the period T3,therefore, the inductor current IL1 becomes a trapezoidal waveform.

[0156] (Fourth Embodiment)

[0157] A power converter in accordance with a fourth embodiment of thepresent invention will be explained based on FIGS. 14-17. A circuitarrangement of the power converter is identical to the first embodiment,and a control system of the control circuit 1 is different from thefirst embodiment. The similar parts of these embodiments are identifiedby the same reference character. The control circuit 1 controls thefirst, second, and third switching elements Q1-Q3 in three differenton/off patterns, as shown in FIG. 14. In a first pattern, all theswitching elements Q1-Q3 are turned on. In a second pattern, only thesecond switching element Q2 is turned on. In a third pattern, all theswitching elements Q1-Q3 are turned off. Each switching element isturned on and off at a frequency higher enough than the frequency of theAC source (50-60 Hz), for example, dozens-several hundred kHz.

[0158]FIGS. 15A-15C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 15A, a current I1 flows increasingly througha closed loop; from the AC source to the AC source via the rectifiercircuit DB, the first switching element Q1, the inductor L, the firstdiode D1, the load LD, the second switching element Q2, the smoothingcapacitor C1, the third switching element Q3, and the rectifier circuitDB. The current I1 is accompanied by a discharging current from thesmoothing capacitor C1. A simplified equivalent circuit in the firstperiod is shown in FIG. 16A, which is a series circuit of the inductor Land the load LD connected across a series circuit of the chargedsmoothing capacitor C1 and a DC source E which outputs an input voltageVin.

[0159] In the second period T2, as shown in FIG. 15B, a current I2 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the first diodeD1, the load LD, the second switching element Q2, the second diode D2,and the third diode D3. The current I2 flows independently of a chargeand a discharge of the smoothing capacitor C1. A simplified equivalentcircuit in this period is shown in FIG. 16B, which is a series circuitof the inductor L and the load LD.

[0160] In the third period T3, as shown in FIG. 15C, a current I3 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the first diodeD1, the load LD, the fourth diode D4, the smoothing capacitor C1, thesecond diode D2, the third diode D3. The current I3 is accompanied by acharging current to the smoothing capacitor C1. A simplified equivalentcircuit in this period is shown in FIG. 16C, which is a series circuitof the inductor L, the load LD, and the smoothing capacitor C1.

[0161] In the first period T1, a first current supplying mode is given,in which the current flows through a closed loop including the ACsource, the inductor L, and the load LD. In the second and third periodsT2 and T3, a second current supplying mode is given, in which thecurrent flows through a closed loop including the inductor L and theload LD but excluding the AC source. That is, the control circuit canimprove a harmonic distortion (a power-factor) and limit the current tothe load, always passing the current to the load and the inductor L, byrepeating the first current supplying mode and the second currentsupplying mode alternately.

[0162] As shown in FIG. 17, the inductor L sees a current IL1 oftrapezoidal waveform over the periods T1 to T3. The period in which theinductor L contributes to the input current from the AC source is onlythe periods T1, and the periods in which the inductor L contributes tothe output current to the load LD are the periods T1-T3.

[0163] If the output voltage of the rectifier circuit DB is Vin, thevoltage across the smoothing capacitor C1 is Vc1, and the voltage acrossthe load LD is VLd, then a voltage VLt1 across the inductor L in thefirst period T1 is expressed as; VLt1=Vin+Vc1−VLd, and a voltage VLt2across the inductor L in the second period T2 is expressed as;VLt2=−VLd, and a voltage VLt3 across the inductor L in the third periodT3 is expressed as; VLt3=−Vc1−VLd. Because Vc1>Vin is always held inthis circuit arrangement, VLt1>VLt2>VLt3 are always held. These voltagesVLt1, VLt2, and VLt3 across the inductor L define a gradient of theinductor current IL1 in the periods T1-T3. Thus, a gradient in theperiod T1>a gradient in the period T2>a gradient in the period T3,therefore, the inductor current IL1 becomes a trapezoidal waveform.

[0164] (Fifth Embodiment)

[0165] A power converter in accordance with a fifth embodiment of thepresent invention will be explained based on FIGS. 18-21. A circuitarrangement of the power converter is identical to the first embodiment,and a control system of the control circuit 1 is different from thefirst embodiment. The similar parts of these embodiments are identifiedby the same reference character. The control circuit 1 controls thefirst, second, and third switching elements Q1-Q3 in three differenton/off patterns, as shown in FIG. 18. In a first pattern, the second andthird switching elements Q2 and Q3 are turned on. In a second pattern,the first and second switching elements Q1 and Q2 are turned on. In athird pattern, all the switching elements Q1-Q3 are turned off. Eachswitching element is turned on and off at a frequency higher enough thanthe frequency of the AC source (50-60 Hz), for example, dozens-severalhundred kHz.

[0166]FIGS. 19A-19C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 19A, a current I1 flows increasingly througha closed loop; from the smoothing capacitor C1 to the smoothingcapacitor C1 via the third switching element Q3, the third diode D3, theinductor L, the first diode D1, the load LD, the second switchingelement Q2. The current I1 is accompanied by a discharging current fromthe smoothing capacitor C1. A simplified equivalent circuit in the firstperiod is shown in FIG. 20A, which is a series circuit of the inductor Land the load LD connected across the charged smoothing capacitor C1.

[0167] In the second period T2, as shown in FIG. 19B, a current I2 flowsincreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first switching element Q1, theinductor L, the first diode D1, the load LD, the second switchingelement Q2, the second diode D2, and the rectifier circuit DB. Thecurrent I2 flows independently of a charge and a discharge of thesmoothing capacitor C1. A simplified equivalent circuit in this periodis shown in FIG. 20B, which is a series circuit of the inductor L andthe load LD connected across the DC source E which outputs an inputvoltage Vin.

[0168] In the third period T3, as shown in FIG. 19C, a current I3 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the first diodeD1, the load LD, the fourth diode D4, the smoothing capacitor C1, thesecond diode D2, the third diode D3. The current I3 is accompanied by acharging current to the smoothing capacitor C1. A simplified equivalentcircuit in this period is shown in FIG. 20C, which is a series circuitof the inductor L, the load LD, and the smoothing capacitor C1.

[0169] In the second period T2, a first current supplying mode is given,in which the current flows through a closed loop including the ACsource, the inductor L, and the load LD. In the first and third periodsT1 and T3, a second current supplying mode is given, in which thecurrent flows through a closed loop including the inductor L and theload LD but excluding the AC source. That is, the control circuit canimprove a harmonic distortion (a power-factor) and limit the current tothe load, always passing the current to the load and the inductor L, byrepeating the first current supplying mode and the second currentsupplying mode alternately.

[0170] As shown in FIG. 21, the inductor L sees a current IL1 oftrapezoidal waveform over the periods T1 to T3. The period in which theinductor L contributes to the input current from the AC source is onlythe periods T2, and the periods in which the inductor L contributes tothe output current to the load LD are the periods T1-T3.

[0171] If the output voltage of the rectifier circuit DB is Vin, thevoltage across the smoothing capacitor C1 is Vc1, and the voltage acrossthe load LD is VLd, then a voltage VLt1 across the inductor L in thefirst period T1 is expressed as; VLt1=Vc1−VLd, and a voltage VLt2 acrossthe inductor L in the second period T2 is expressed as; VLt2=Vin−VLd,and a voltage VLt3 across the inductor L in the third period T3 isexpressed as; VLt3=−Vc1−VLd. Because Vc1>Vin is always held in thiscircuit arrangement, VLt1>VLt2>VLt3 are always held. These voltagesVLt1, VLt2, and VLt3 across the inductor L define a gradient of theinductor current IL1 in the periods T1-T3. Thus, a gradient in theperiod T1>a gradient in the period T2>a gradient in the period T3,therefore, the inductor current IL1 becomes a trapezoidal waveform.

[0172] (Sixth Embodiment)

[0173] A power converter in accordance with a sixth embodiment of thepresent invention will be explained based on FIGS. 22-25. A circuitarrangement of the power converter is identical to the first embodiment,and a control system of the control circuit 1 is different from thefirst embodiment. The similar parts of these embodiments are identifiedby the same reference character. The control circuit 1 controls thefirst, second, and third switching elements Q1-Q3 in three differenton/off patterns, as shown in FIG. 22. In a first pattern, the second andthird switching elements Q2 and Q3 are turned on. In a second pattern,only the second switching element Q2 is turned on. In a third pattern,only the first switching element Q1 is turned on. Each switching elementis turned on and off at a frequency higher enough than the frequency ofthe AC source (50-60 Hz), for example, dozens-several hundred kHz.

[0174]FIGS. 23A-23C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 23A, a current I1 flows increasingly througha closed loop; from the smoothing capacitor C1 to the smoothingcapacitor C1 via the third switching element Q3, the third diode D3, theinductor L, the first diode D1, the load LD, the second switchingelement Q2. The current I1 is accompanied by a discharging current fromthe smoothing capacitor C1. A simplified equivalent circuit in the firstperiod is shown in FIG. 24A, which is a series circuit of the inductor Land the load LD connected across the charged smoothing capacitor C1.

[0175] In the second period T2, as shown in FIG. 23B, a current I2 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the first diodeD1, the load LD, the second switching element Q2, the second diode D2,the third diode D3. The current I2 flows independently of a charge and adischarge of the smoothing capacitor C1. A simplified equivalent circuitin this period is shown in FIG. 24B, which is a series circuit of theinductor L and the load LD.

[0176] In the third period T3, as shown in FIG. 23C, a current I3 flowsdecreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first switching element Q1, theinductor L, the first diode D1, the load LD, the fourth diode D4, thesmoothing capacitor C1, the second diode D2, and the rectifier circuitDB. The current I3 is accompanied by a charging current to the smoothingcapacitor C1. A simplified equivalent circuit in this period is shown inFIG. 24C, which is a series circuit of the inductor L, the load LD, andthe smoothing capacitor C1 connected across the DC source E.

[0177] In the third period T3, a first current supplying mode is given,in which the current flows through a closed loop including the ACsource, the inductor L, and the load LD. In the first and second periodsT1 and T2, a second current supplying mode is given, in which thecurrent flows through a closed loop including the inductor L and theload LD but excluding the AC source. That is, the control circuit canimprove a harmonic distortion (a power-factor) and limit the current tothe load, always passing the current to the load and the inductor L, byrepeating the first current supplying mode and the second currentsupplying mode alternately.

[0178] As shown in FIG. 25, the inductor L sees a current IL1 oftrapezoidal waveform over the periods T1 to T3. The period in which theinductor L contributes to the input current from the AC source is onlythe periods T3, and the periods in which the inductor L contributes tothe output current to the load LD are the periods T1-T3.

[0179] If the output voltage of the rectifier circuit DB is Vin, thevoltage across the smoothing capacitor C1 is Vc1, and the voltage acrossthe load LD is VLd, then a voltage VLt1 across the inductor L in thefirst period T1 is expressed as; VLt1=Vc1−VLd, and a voltage VLt2 acrossthe inductor L in the second period T2 is expressed as; VLt2=−VLd, and avoltage VLt3 across the inductor L in the third period T3 is expressedas; VLt3=Vin−Vc1−VLd. Because Vc1>Vin is always held in this circuitarrangement, VLt1>VLt2>VLt3 are always held. These voltages VLt1, VLt2,and VLt3 across the inductor L define a gradient of the inductor currentIL1 in the periods T1-T3. Thus, a gradient in the period T1>a gradientin the period T2>a gradient in the period T3, therefore, the inductorcurrent IL1 becomes a trapezoidal waveform.

[0180] (Seventh Embodiment)

[0181] A power converter in accordance with a seventh embodiment of thepresent invention will be explained based on FIGS. 26-28. This powerconverter is designed to convert AC power from an AC source into DCpower and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes a rectifier circuit DB which rectifies the AC currentfrom the AC source to give a DC voltage, five switching elements Q1-Q5,one inductor L, and one smoothing capacitor C1. The first switchingelement Q1 and the second switching element Q2 are connected in serieswith a first diode across the rectifier circuit DB, and the first diodeD1 is inserted between a high voltage side of the rectifier circuit DBand the first switching element Q1, and a cathode of the first diode D1is connected to the first switching element Q1. The third switchingelement Q3 and the fourth switching element Q4 are connected in serieswith a second diode across the rectifier circuit DB, and the seconddiode D2 is inserted between a high voltage side of the rectifiercircuit DB and the third switching element Q3, and a cathode of thesecond diode D2 is connected to the third switching element Q3. Thesecond switching element Q2 and the fourth switching element Q4 areconnected through a common third diode D3 to a low voltage side of therectifier circuit DB. An inductor L is connected in series with the loadLD between the connection point of the first switching element Q1 withthe second switching element Q2 and the connection point of the thirdswitching element Q3 with the fourth switching element Q4. The inductorL, the load LD, and a fourth diode D4 are connected in series with thesmoothing capacitor C1 across the second switching element Q2, and theload LD, the inductor L, and a fifth diode D5 are connected in seriesacross the fourth switching element Q4. The fifth switching element Q5is connected in series with the smoothing capacitor C1 across therectifier circuit DB. Each of the second switching element Q2 and fourthswitching element Q4 is FET, and a parasitic diode of each FET forms abypass allowing a reverse current to flow across each switching element.The other switching elements are also FETs, but they are not necessarilylimited to FETs.

[0182] The control circuit passes the current to the load in onedirection by making the first and fourth switching elements Q1, Q4 turnon and off at the same time and making both the second and thirdswitching elements Q2, Q3 turn off in the meantime, while making thefifth switching element Q5 turn on and off on a predetermined cycle. Andalso, the control circuit passes the current to the load in the reversedirection by making the second and third switching elements Q2, Q3 turnon and off at the same time and making both the first and fourthswitching elements Q1, Q4 turn off in the meantime, while making thefifth switching element Q5 turn on and off on a predetermined cycle. Bythis, the control circuit can drive the load by the alternating currentof low frequency.

[0183] The control circuit 1 can always pass the current to both theload and the inductor, improving the harmonic distortion, namely, apower-factor, by controlling the five switching elements Q1-Q5 in sixdifferent patterns. These six patterns are classified into a positivecycle in which three continuous patterns are repeated, and a negativecycle in which remaining three continuous patterns are repeated. Each ofthe positive cycle and the negative cycle is repeated alternately at alow frequency, for example, at 100 Hz. FIG. 27 shows a control system tocontrol the first, fourth, and fifth switching elements Q1, Q4, Q5 inthree different patterns in the positive cycle. In a control system inthe negative cycle, the common switching element Q5 and the remainingswitching elements Q2, Q3 are controlled; in more detail, the switchingelement Q5 is controlled like in the positive cycle and the second andthird switching elements Q2, Q3 are controlled like the first and fourthswitching elements Q1, Q4 in the positive cycle. In each cycle, thethree patterns are repeated two or more times in the half cycle of theAC current from the AC source.

[0184] In a first pattern, the first, fourth, and fifth switchingelements Q1, Q4, and Q5 are turned on. In a second pattern, the firstand fourth switching elements Q1, Q4 are turned on. In a third pattern,all the switching elements are turned off. Each switching element isturned on and off at a frequency higher enough than the frequency of theAC source (50-60 Hz), for example, dozens-several hundred kHz.

[0185]FIGS. 28A-28C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 28A, a current I1 flows increasingly througha closed loop; from the AC source to the AC source via the rectifiercircuit DB, the first diode D1, the first switching element Q1, theinductor L, the load LD, the fourth switching element Q4, the smoothingcapacitor C1, the fifth switching element Q5, and the rectifier circuitDB. The current I1 is accompanied by a discharging current from thesmoothing capacitor C1. A simplified equivalent circuit in the firstperiod is shown in FIG. 4A, which is a series circuit of the inductor Land the load LD connected across a series circuit of the chargedsmoothing capacitor C1 and a DC source E which outputs an input voltageVin.

[0186] In the second period T2, as shown in FIG. 28B, a current I2 flowsincreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first diode D1, the first switchingelement Q1, the inductor L, the load LD, the fourth switching elementQ4, the third diode D3, and the rectifier circuit DB. The current I2flows independently of a charge and a discharge of the smoothingcapacitor C1. A simplified equivalent circuit in this period is shown inFIG. 4B, which is a series circuit of the inductor L and the load LDconnected across the DC source E.

[0187] In the third period T3, as shown in FIG. 28C, a current I3 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the load LD, thefourth diode D4, the smoothing capacitor C1, and the bypass of theswitching element Q2. The current I3 is accompanied by a chargingcurrent to the smoothing capacitor C1. A simplified equivalent circuitin this period is shown in FIG. 4C, which is a series circuit of theinductor L, the load LD, and the smoothing capacitor C1.

[0188] In the first and second period T1 and T2, a first currentsupplying mode is given, in which the current flows through a loopincluding the AC source, the inductor L, and the load LD. In the thirdperiod T3, a second current supplying mode is given, in which thecurrent flows through a loop including the inductor L and the load LDbut excluding the AC source. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0189] The control circuit can pass the current of rectangular wave oflow frequency to the load LD by repeating the remaining three patternsof the negative cycle after repeating the first, second, and thirdpatterns two or more times as mentioned above. The control system in thenegative cycle is designed to pass the current to the load in thereverse direction by controlling the second and third switching elementslike the first and fourth switching elements, instead of controlling thefirst and fourth switching elements.

[0190] As is clear from the above operation, this power converter inthis embodiment can convert AC power into DC power and convert the DCpower into AC power at the same time to supply the alternating currentto the load by using only five switching elements Q1-Q5.

[0191] In the third period in the negative cycle, the current flows inthe opposite direction of the direction indicated by the arrow of FIG.28C; from the inductor L to the load via the fifth diode D5, thesmoothing capacitor C1, and the bypass of the fourth switching elementsQ4.

[0192] (Eighth Embodiment)

[0193] A power converter in accordance with an eighth embodiment of thepresent invention is shown in FIGS. 29, 30. A circuit arrangement of thepower converter is identical to the seventh embodiment, and a controlsystem of the control circuit 1 is different from the seventhembodiment. The similar parts of these embodiments are identified by thesame reference character. The control circuit 1 controls the first,second, third, fourth, and fifth switching elements Q1-Q5 in sixdifferent on/off patterns as shown in FIG. 29 to always pass the currentto the load and the inductor, improving the harmonic distortion, namely,a power-factor. These six patterns are classified into a positive cyclein which three continuous patterns are repeated, and a negative cycle inwhich remaining three continuous patterns are repeated. Each of thepositive cycle and the negative cycle is repeated alternately at a lowfrequency, for example, at 100 Hz. FIG. 29 shows a control system tocontrol the first, fourth, and fifth switching elements Q1, Q4, Q5 inthree different patterns in the positive cycle. In a control system inthe negative cycle, the common switching element Q5 and the remainingswitching elements Q2, Q3 are controlled; in more detail, the switchingelement Q5 is controlled like in the positive cycle, and the second andthird switching elements Q2, Q3 are controlled like the first and fourthswitching elements Q1, Q4 in the positive cycle. In each cycle, thethree patterns are repeated two or more times in the half cycle of theAC current from the AC source.

[0194] In a first pattern, the first, fourth, and fifth switchingelements Q1, Q4, and Q5 are turned on. In a second pattern, only thefourth switching element Q4 is turned on. In a third pattern, only thefirst switching elements Q1 is turned on. Each switching element isturned on and off at a frequency higher enough than the frequency of theAC source (50-60 Hz), for example, dozens-several hundred kHz.

[0195]FIGS. 30A-30C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 30A, a current I1 flows increasingly througha closed loop; from the AC source to the AC source via the rectifiercircuit DB, the first diode D1, the first switching element Q1, theinductor L, the load LD, the fourth switching element Q4, the smoothingcapacitor C1, the fifth switching element Q5, and the rectifier circuitDB. The current I1 is accompanied by a discharging current from thesmoothing capacitor C1. A simplified equivalent circuit in the firstperiod is shown in FIG. 8A, which is a series circuit of the inductor Land the load LD connected across a series circuit of the chargedsmoothing capacitor C1 and a DC source E which outputs an input voltageVin.

[0196] In the second period T2, as shown in FIG. 30B, a current I2 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the load LD, thefourth switching element Q4, and the bypass of the second switchingelement Q2. The current I2 flows independently of a charge and adischarge of the smoothing capacitor C1. A simplified equivalent circuitin this period is shown in FIG. 8B, which is a series circuit of theinductor L and the load LD.

[0197] In the third period T3, as shown in FIG. 30C, a current I3 flowsdecreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first diode D1, the first switchingelement Q1, the inductor L, the load LD, the fourth diode D4, thesmoothing capacitor C1, the third diode D3, and the rectifier circuitDB. The current I3 is accompanied by a charging current to the smoothingcapacitor C1. A simplified equivalent circuit in this period is shown inFIG. 8C, which is a series circuit of the inductor L, the load LD, andthe smoothing capacitor C1 connected across the DC source E.

[0198] In the first and third period T1 and T3, a first currentsupplying mode is given, in which the current flows through a closedloop including the AC source, the inductor L, and the load LD. In thesecond period T2, a second current supplying mode is given, in which thecurrent flows through a closed loop including the inductor L and theload LD but excluding the AC source. That is, the control circuit canimprove a harmonic distortion (a power-factor) and limit the current tothe load, always passing the current to the load and the inductor L, byrepeating the first current supplying mode and the second currentsupplying mode alternately.

[0199] The control circuit can pass the current of rectangular wave oflow frequency to the load LD by repeating the remaining three patternsof the negative cycle after repeating the first, second, and thirdpatterns two or more times as mentioned above. The control system in thenegative cycle is designed to pass the current to the load in thereverse direction by controlling the third and second switching elementslike the first and fourth switching elements, respectively, instead ofcontrolling the first and fourth switching elements.

[0200] (Ninth Embodiment)

[0201] A power converter in accordance with a ninth embodiment of thepresent invention is shown in FIGS. 31, 32. A circuit arrangement of thepower converter is identical to the seventh embodiment, and a controlsystem of the control circuit 1 is different from the seventhembodiment. The similar parts of these embodiments are identified by thesame reference character. The control circuit 1 controls the first,second, third, fourth, and fifth switching elements Q1-Q5 in sixdifferent on/off patterns as shown in FIG. 31 to always pass the currentto the load and the inductor, improving the harmonic distortion, namely,a power-factor. These six patterns are classified into a positive cyclein which three continuous patterns are repeated, and a negative cycle inwhich remaining three continuous patterns are repeated. Each of thepositive cycle and the negative cycle is repeated alternately at a lowfrequency, for example, at 100 Hz. FIG. 31 shows a control system tocontrol the first, fourth, and fifth switching elements Q1, Q4, Q5 inthree different patterns in the positive cycle. In a control system inthe negative cycle, the common switching element Q5 and the remainingswitching elements Q2, Q3 are controlled; in more detail, the switchingelement Q5 is controlled like in the positive cycle, and the third andsecond switching element Q3, Q2 are controlled like the first and fourthswitching elements Q1, Q4 in the positive cycle, respectively. In eachcycle, the three patterns are repeated two or more times in the halfcycle of the AC current from the AC source.

[0202] In a first pattern, the first, fourth, and fifth switchingelements Q1, Q4, and Q5 are turned on. In a second pattern, only thefourth switching element Q4 is turned on. In a third pattern, all theswitching elements are turned off. Each switching element is turned onand off at a frequency higher enough than the frequency of the AC source(50-60 Hz), for example, dozens-several hundred kHz.

[0203]FIGS. 32A-32C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 32A, a current I1 flows increasingly througha closed loop; from the AC source to the AC source via the rectifiercircuit DB, the first diode D1, the first switching element Q1, theinductor L, the load LD, the fourth switching element Q4, the smoothingcapacitor C1, the fifth switching element Q5, and the rectifier circuitDB. The current I1 is accompanied by a discharging current from thesmoothing capacitor C1. A simplified equivalent circuit in the firstperiod is shown in FIG. 16A, which is a series circuit of the inductor Land the load LD connected across a series circuit of the chargedsmoothing capacitor C1 and a DC source E which outputs an input voltageVin.

[0204] In the second period T2, as shown in FIG. 32B, a current I2 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the load LD, thefourth switching element Q4, and the bypass of the second switchingelement Q2. The current I2 flows independently of a charge and adischarge of the smoothing capacitor C1. A simplified equivalent circuitin this period is shown in FIG. 16B, which is a series circuit of theinductor L and the load LD.

[0205] In the third period T3, as shown in FIG. 32C, a current I3 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the load LD, thefourth diode D4, the smoothing capacitor C1, the bypass of the secondswitching element Q2. The current I3 is accompanied by a chargingcurrent to the smoothing capacitor C1. A simplified equivalent circuitin this period is shown in FIG. 16C, which is a series circuit of theinductor L, the load LD, and the smoothing capacitor C1.

[0206] In the first period T1, a first current supplying mode is given,in which the current flows through a closed loop including the ACsource, the inductor L, and the load LD. In the second and third periodsT2, T3, a second current supplying mode is given, in which the currentflows through a closed loop including the inductor L and the load LD butexcluding the AC source. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor L, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0207] The control circuit can pass the current of rectangular wave oflow frequency to the load LD by repeating the remaining three patternsof the negative cycle after repeating the first, second, and thirdpatterns two or more times as mentioned above. The control system in thenegative cycle is designed to pass the current to the load in thereverse direction by controlling the third and second switching elementslike the first and fourth switching elements, respectively, instead ofcontrolling the first and fourth switching elements.

[0208] (Tenth Embodiment)

[0209] A power converter in accordance with a tenth embodiment of thepresent invention will be explained based on FIGS. 33-35. This powerconverter is designed to convert AC power from an AC source into DCpower and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes a rectifier circuit DB which rectifies the AC currentfrom the AC source to give a DC voltage, five switching elements Q1-Q5,one inductor L, and one smoothing capacitor C1. The first switchingelement Q1 and the second switching element Q2 are connected in serieswith a first diode D1 across the rectifier circuit DB, and the firstdiode D1 is inserted between a high voltage side of the rectifier DB andthe first switching element Q1. A cathode of the first diode D1 isconnected to the first switching element Q1. The third switching elementQ3 and the fourth switching element Q4 are connected in series with asecond diode D2 across the rectifier circuit DB, and the second diode D2is inserted between a high voltage side of the rectifier DB and thethird switching element Q3. A cathode of the second diode D2 isconnected to the third switching element Q3. The inductor L is connectedin series with the load LD, and the series circuit is inserted betweenthe connection point of the first switching element Q1 with the secondswitching element Q2 and the connection point of the third switchingelement Q3 with the fourth switching element Q4. The third diode D3 isconnected in series with the smoothing capacitor C1 across the fourthswitching element Q4. The fourth diode D4 is connected in series withthe smoothing capacitor C1 across the second switching element Q2. Aseries circuit of the fifth switching element Q5, the first diode D1,the first switching element Q1, the inductor L, the load LD, and thefourth switching element Q4 is connected across the smoothing capacitorC1. Also, a series circuit of the fifth switching element Q5, the seconddiode D2, the third switching element Q3, the inductor L, the load LD,and the second switching element Q2 is connected across the smoothingcapacitor.

[0210] The control circuit passes the current to the load in onedirection by making the first and fourth switching elements Q1, Q4 turnon and off at the same time and making both the second and thirdswitching elements Q2, Q3 turn off in the meantime, while making thefifth switching element Q5 turn on and off on a predetermined cycle. Andalso, the control circuit passes the current to the load in the reversedirection by making the second and third switching elements Q2, 03 turnon and off at the same time and making both the first and fourthswitching elements Q1, Q4 turn off, while making the fifth switchingelement Q5 turn on and off on a predetermined cycle. By this, thecontrol circuit can drive the load by the alternating current of lowfrequency.

[0211] The control circuit 1 can always pass the current to both theload and the inductor, improving a harmonic distortion, namely, apower-factor, by controlling the five switching elements Q1-Q5 in sixdifferent patterns. These six patterns are classified into a positivecycle in which three continuous patterns are repeated, and a negativecycle in which remaining three continuous patterns are repeated. Each ofthe positive cycle and the negative cycle is repeated alternately at alow frequency, for example, at 100 Hz. FIG. 34 shows a control system tocontrol the first, fourth, and fifth switching elements Q1, Q4, Q5 inthree different patterns in the positive cycle. In a control system inthe negative cycle, the common switching element Q5 and the remainingswitching elements Q2, Q3 are controlled; in more detail, the switchingelement Q5 is controlled like in the positive cycle, and the third andsecond switching element Q3 and Q2 are controlled like the first andfourth switching elements Q1, Q4 in the positive cycle, respectively. Ineach cycle, the three patterns are repeated two or more times in thehalf cycle of the AC current from the AC source.

[0212] In a first pattern, the first, fourth, and fifth switchingelements Q1, Q4, and Q5 are turned on. In a second pattern, the firstand fourth switching elements Q1, Q4 are turned on. In a third pattern,all the switching elements except the first switching element are turnedoff. Each switching element is turned on and off at a frequency higherenough than the frequency of the AC source (50-60 Hz), for example,dozens-several hundred kHz.

[0213]FIGS. 35A-35C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 35A, a current I1 flows increasingly througha closed loop; from the smoothing capacitor C1 to the smoothingcapacitor C1 via the fifth switching element Q5, the first diode D1, thefirst switching element Q1, the inductor L, the load LD, and the fourthswitching element Q4. The current I1 is accompanied by a dischargingcurrent from the smoothing capacitor C1. A simplified equivalent circuitin the first period is shown in FIG. 12A, which is a series circuit ofthe inductor L and the load LD connected across the charged smoothingcapacitor C1.

[0214] In the second period T2, as shown in FIG. 35B, a current I2 flowsincreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first diode D1, the first switchingelement Q1, the inductor L, the load LD, the fourth switching elementQ4, and the rectifier circuit DB. The current I2 flows independently ofa charge and a discharge of the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 12B, which is aseries circuit of the inductor L and the load LD connected across the DCsource E which outputs an input voltage Vin.

[0215] In the third period T3, as shown in FIG. 35C, a current I3 flowsdecreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first diode D1, the first switchingelement Q1, the inductor L, the load LD, the third diode D3, thesmoothing capacitor C1, and the rectifier circuit DB. The current I3 isaccompanied by a charging current to the smoothing capacitor C1. Asimplified equivalent circuit in this period is shown in FIG. 12C, whichis a series circuit of the inductor L, the load LD, and the smoothingcapacitor C1 connected across the DC source E.

[0216] In the second and third period T2 and T3, a first currentsupplying mode is given, in which the current flows through a closedloop including the AC source, the inductor L, and the load LD. In thefirst period T1, a second current supplying mode is given, in which thecurrent flows through a closed loop including the inductor L and theload LD but excluding the AC source. That is, the control circuit canimprove a harmonic distortion (a power-factor) and limit the current tothe load, always passing the current to the load and the inductor L, byrepeating the first current supplying mode and the second currentsupplying mode alternately.

[0217] The control circuit can pass the current of rectangular wave oflow frequency to the load LD by repeating the remaining three patternsof the negative cycle after repeating the first, second, and thirdpatterns two or more times as mentioned above. The control system in thenegative cycle is designed to pass the current to the load in thereverse direction by controlling the third and second switching elementslike the first and fourth switching elements in the positive cycle,instead of controlling the first and fourth switching elements.

[0218] In the third period in the negative cycle, the current flows inthe opposite direction of the direction indicated by the arrow of FIG.35C; from the inductor L to the load via the fourth diode D4, thesmoothing capacitor C1, and the bypass of the fourth switching elementQ4.

[0219] (Eleventh embodiment)

[0220] A power converter in accordance with an eleventh embodiment ofthe present invention is shown in FIGS. 36, 37. A circuit arrangement ofthe power converter is identical to the tenth embodiment, and a controlsystem of the control circuit 1 is different from the tenth embodiment.The similar parts of these embodiments are identified by the samereference character. The control circuit 1 controls the first, second,third, fourth, and fifth switching elements Q1-Q5 in six differenton/off patterns as shown in FIG. 36 to always pass the current to theload and the inductor, improving the harmonic distortion, namely, apower-factor. These six patterns are classified into a positive cycle inwhich three continuous patterns are repeated, and a negative cycle inwhich remaining three continuous patterns are repeated. Each of thepositive cycle and the negative cycle is repeated alternately at a lowfrequency, for example, at 100 Hz. FIG. 36 shows a control system tocontrol the first, fourth, and fifth switching elements Q1, Q4, and Q5in three different patterns in the positive cycle. In a control systemin the negative cycle, the common switching element Q5 and the remainingswitching elements Q2, Q3 are controlled; in more detail, the switchingelement Q5 is controlled like in the positive cycle, and the third andsecond switching elements Q3, Q2 are controlled like the first andfourth switching elements Q1, Q4 in the positive cycle. In each cycle,the three patterns are repeated two or more times in the half cycle ofthe AC current from the AC source.

[0221] In a first pattern, the first, fourth, and fifth switchingelements Q1, 04, and Q5 are turned on. In a second pattern, the firstand fourth switching elements Q1, Q4 are turned on. In a third pattern,all the switching elements Q1-Q5 are turned off. Each switching elementis turned on and off at a frequency higher enough than the frequency ofthe AC source (50-60 Hz), for example, dozens-several hundred kHz.

[0222]FIGS. 37A-37C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 37A, a current I1 flows increasingly througha closed loop; from the smoothing capacitor C1 to the smoothingcapacitor C1 via the fifth switching element Q5, the first diode D1, thefirst switching element Q1, the inductor L, the load LD, the fourthswitching element Q4. The current I1 is accompanied by a dischargingcurrent from the smoothing capacitor C1. A simplified equivalent circuitin this period is shown in FIG. 20A, which is a series circuit of theinductor L and the load LD connected across the charged smoothingcapacitor C1.

[0223] In the second period T2, as shown in FIG. 37B, a current I2 flowsincreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first diode D1, the first switchingelement Q1, the inductor L, the load LD, the fourth switching elementQ4, and the rectifier circuit DB. The current I2 flows independently ofa charge and a discharge of the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 20B, which is aseries circuit of the inductor L and the load LD connected across the DCsource E which outputs an input voltage Vin.

[0224] In the third period T3, as shown in FIG. 37C, a current I3 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the load LD, thethird diode D3, the smoothing capacitor C1, and the bypass of the secondswitching element Q2. The current I3 is accompanied by a chargingcurrent to the smoothing capacitor C1. A simplified equivalent circuitin this period is shown in FIG. 20C, which is a series circuit of theinductor L, the load LD, and the smoothing capacitor C1.

[0225] In the second period T2, a first current supplying mode is given,in which the current flows through a closed loop including the ACsource, the inductor L, and the load LD. In the first and third periodsT1 and T3, a second current supplying mode is given, in which thecurrent flows through a closed loop including the inductor L and theload LD but excluding the AC source. That is, the control circuit canimprove a harmonic distortion (a power-factor) and limit the current tothe load, always passing the current to the load and the inductor L, byrepeating the first current supplying mode and the second currentsupplying mode alternately.

[0226] The control circuit can pass the current of rectangular wave oflow frequency to the load LD by repeating the remaining three patternsof the negative cycle after repeating the first, second, and thirdpatterns two or more times as mentioned above. The control system in thenegative cycle is designed to pass the current to the load in thereverse direction by controlling the third and second switching elementslike the first and fourth switching elements, respectively, instead ofcontrolling the first and fourth switching elements.

[0227] In the third period in the negative cycle, the current flows inthe opposite direction of the direction indicated by the arrow of FIG.37C; from the inductor L to the load via the fourth diode D4, thesmoothing capacitor C1, and the bypass of the fourth switching elementQ4.

[0228] (Twelfth Embodiment)

[0229] A power converter in accordance with a twelfth embodiment of thepresent invention is shown in FIGS. 38, 39. A circuit arrangement of thepower converter is identical to the tenth embodiment, and a controlsystem of the control circuit 1 is different from the tenth embodiment.The similar parts of these embodiments are identified by the samereference character. The control circuit 1 controls the first, second,third, fourth, and fifth switching elements Q1-Q5 in six differenton/off patterns as shown in FIG. 38 to always pass the current to theload and the inductor, improving the harmonic distortion, namely, apower-factor. These six patterns are classified into a positive cycle inwhich three continuous patterns are repeated, and a negative cycle inwhich remaining three continuous patterns are repeated. Each of thepositive cycle and the negative cycle is repeated alternately at a lowfrequency, for example, at 100 Hz. FIG. 38 shows a control system tocontrol the first, fourth, and fifth switching elements Q1, Q4, Q5 inthree different patterns in the positive cycle. In a control system inthe negative cycle, the common switching element Q5 and the remainingswitching elements Q2, Q3 are controlled; in more detail, the switchingelement Q5 is controlled like in the positive cycle, and the third andsecond switching elements Q3, Q2 are controlled like the first andfourth switching elements Q1, Q4 in the positive cycle, respectively. Ineach cycle, the three patterns are repeated two or more times in thehalf cycle of the AC current from the AC source.

[0230] In a first pattern, the first, fourth, and fifth switchingelements Q1, Q4, and Q5 are turned on. In a second pattern, only thefourth switching element Q4 is turned on. In a third pattern, only thefirst switching element Q1 is turned on. Each switching element isturned on and off at a frequency higher enough than the frequency of theAC source (50-60 Hz), for example, dozens-several hundred kHz.

[0231]FIGS. 39A-39C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 39A, a current I1 flows increasingly througha closed loop; from the smoothing capacitor C1 to the smoothingcapacitor C1 via the fifth switching element Q5, the first diode D1, thefirst switching element Q1, the inductor L, the load LD, the fourthswitching element Q4. The current I1 is accompanied by a dischargingcurrent from the smoothing capacitor C1. A simplified equivalent circuitin the first period is shown in FIG. 24A, which is a series circuit ofthe inductor L and the load LD connected across the charged smoothingcapacitor C1.

[0232] In the second period T2, as shown in FIG. 39B, a current I2 flowsdecreasingly by back electromotive force of the inductor L through aclosed loop; from the inductor L to the inductor L via the load LD, thefourth switching element Q4, and the bypass of the second switchingelement Q2. The current I2 flows independently of a charge and adischarge of the smoothing capacitor C1. A simplified equivalent circuitin this period is shown in FIG. 24B, which is a series circuit of theinductor L and the load LD.

[0233] In the third period T3, as shown in FIG. 39C, a current I3 flowsdecreasingly through a closed loop; from the AC source to the AC sourcevia the rectifier circuit DB, the first diode D1, the first switchingelement Q1, the inductor L, the load LD, the third diode D3, thesmoothing capacitor C1, and the rectifier circuit DB. The current I3 isaccompanied by a charging current to the smoothing capacitor C1. Asimplified equivalent circuit in this period is shown in FIG. 24C, whichis a series circuit of the inductor L, the load LD, and the smoothingcapacitor C1 connected across the DC source E.

[0234] In the third period T3, a first current supplying mode is given,in which the current flows through a closed loop including the ACsource, the inductor L, and the load LD. In the first and second periodsT1 and T2, a second current supplying mode is given, in which thecurrent flows through a closed loop including the inductor L and theload LD but excluding the AC source. That is, the control circuit canimprove a harmonic distortion (a power-factor) and limit the current tothe load, always passing the current to the load and the inductor L, byrepeating the first current supplying mode and the second currentsupplying mode alternately.

[0235] The control circuit can pass the current of rectangular wave oflow frequency to the load LD by repeating the remaining three patternsof the negative cycle after repeating the first, second, and thirdpatterns two or more times as mentioned above. The control system in thenegative cycle is designed to pass the current to the load in thereverse direction by controlling the third and second switching elementslike the first and fourth switching elements, respectively, instead ofcontrolling the first and fourth switching elements.

[0236] In the third period in the negative cycle, the current flows inthe opposite direction of the direction indicated by the arrow of FIG.39C; from the inductor L to the load via the fourth diode D4, thesmoothing capacitor C1, and the bypass of the fourth switching elementQ4.

[0237] (Thirteenth Embodiment)

[0238] A power converter in accordance with a thirteenth embodiment ofthe present invention will be explained based on FIGS. 40-43. This powerconverter is designed to convert AC power from an AC source into DCpower and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes four switching elements Q1-Q4, one inductor L, andtwo smoothing capacitors C1, C2. The first switching element Q1 and thesecond switching element Q2 are connected in series with the inductor Land the load LD across the AC source, and the first switching element Q1and the third switching element Q3 are connected in series with theinductor L and the load LD across the AC source. A series circuit of afirst smoothing capacitor C1 and a second smoothing capacitor C2 isconnected across a series circuit of the second switching element Q2 andthe third switching element Q3. A series circuit of a first diode D1 anda second diode D2 are connected across a series circuit of the secondswitching element Q2 and the third switching element Q3. A diode bridgeD11-D14 is inserted between the connection point of the first smoothingcapacitor C1 with the second smoothing capacitor C2 and the AC source,each input terminal of the diode bridge is connected to the connectionpoint of the first smoothing capacitor C1 with the second smoothingcapacitor C2 and the AC source, respectively. The first switchingelement Q1 is connected between output terminals of the diode bridgeD11-D14. A series circuit of a third diode D3 and a fourth diode D4 isconnected across a series circuit of the first diode D1 and the seconddiode D2. The inductor L and the load LD are connected in series betweenthe connection point of the first diode D1 with the second diode D2 andthe connection point of the third diode D3 with the fourth diode D4. Thefourth switching element Q4 is connected across a series circuit of thethird diode D3 and the fourth diode D4. A series circuit of a fifthdiode D5 and a sixth diode D6 is connected across a series circuit ofthe first diode D1 and the second diode D2, and the AC source isinserted between the connection point of the first diode D1 with thesecond diode D2 and the connection point of the fifth diode D5 with thesixth diode D6. The second and third switching elements Q2 and Q3, eachdefined by FET, have a parasitic diode, respectively, which defines abypass allowing a reverse current to flow across each switching element.

[0239] The control circuit passes the current to the load in onedirection by making the second switching element Q2 turn on and off andkeeping the third switching element Q3 turned off in the meantime, whilemaking the first and fourth switching elements Q1, Q4 turn on and offalternately, and also, the control circuit passes the current to theload in the reverse direction by making the third switching element Q3turn on and off and keeping the second switching element turned off inthe meantime, while making the first and fourth switching elements Q1,Q4 turn on and off alternately.

[0240] The control circuit 1 can always pass the current to the load andthe inductor, improving the harmonic distortion, namely, a power-factor,by controlling the first, second, third, and fourth switching elementsQ1-Q4 to turn on and off in six different patterns. These six patternsare classified into a positive cycle in which three continuous patternsare repeated in a positive half cycle of the AC source, and a negativecycle in which remaining three continuous patterns are repeated in anegative half cycle of the AC source. Each of the positive cycle and thenegative cycle is repeated alternately at a low frequency, for example,at 100 Hz. FIG. 41 shows a control system to control the switchingelements Q1-Q4.

[0241] In a first pattern in the positive cycle, the first and secondswitching elements Q1, Q2 are turned on. In a second pattern, only thefourth switching element Q4 is turned on. In a third pattern, all theswitching elements Q1-Q4 are turned off. In a first pattern in thenegative cycle, the first and third switching elements Q1, Q3 are turnedon, and in a second pattern, only the fourth switching element Q4 isturned on, and in a third pattern, all the switching elements Q1-Q4 areturned off. Each switching element is turned on and off at a frequencyhigher enough than the frequency of the AC source (50-60 Hz), forexample, dozens several hundred kHz.

[0242]FIGS. 42A-42C show a current which flows through the circuit in afirst period T1p controlled based on the first pattern in the positivecycle, in a second period T2p controlled based on the second pattern inthe positive cycle, and in a third period T3p controlled based on thethird pattern in the positive cycle, respectively. In the first periodT1p, as shown in FIG. 42A, a current I1 p flows increasingly through aclosed loop; from the AC source to the AC source via the diode D13, thefirst switching element Q1, the diode D12, the smoothing capacitor C1,the second switching element Q2, the load LD, and the inductor L. Thecurrent I1 p is accompanied by a discharging current from the smoothingcapacitor C1. A simplified equivalent circuit in this period is shown inFIG. 4A.

[0243] In the second period T2p, as shown in FIG. 42B, a current I2 pflows increasingly through a closed loop; from the AC source to the ACsource via the fifth diode D5, the fourth switching element Q4, thefourth diode D4, the load LD, and the inductor L. The current I2 p flowsindependently of a charge and a discharge of the smoothing capacitorsC1, C2. A simplified equivalent circuit in this period is shown in FIG.4B.

[0244] In the third period T3p, as shown in FIG. 42C, a current I3 pflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the first diodeD1, the smoothing capacitor C1, the smoothing capacitor C2, theparasitic diode of the third switching element Q3, and the load LD. Thecurrent I3 p is accompanied by a charging current to the smoothingcapacitors C1, C2. A simplified equivalent circuit in this period is acircuit in which the smoothing capacitor C1 of FIG. 4C was replaced bythe series circuit of the smoothing capacitors C1 and C2.

[0245]FIGS. 43A-43C show a current which flows through the circuit in afirst period T1n controlled based on the first pattern in the negativecycle, in a second period T2n controlled based on the second pattern inthe negative cycle, and in a third period T3n controlled based on thethird pattern in the negative cycle, respectively. In the first periodT1n, as shown in FIG. 43A, a current I1 n flows increasingly through aclosed loop; from the AC source to the AC source via the inductor L, theload LD, the third switching element Q3, the smoothing capacitor C2, thediode D11, the first switching element Q1, and the diode D14. Thecurrent I1 n is accompanied by a discharging current from the smoothingcapacitor C2. A simplified equivalent circuit in this period is acircuit in which the smoothing capacitor C1 of FIG. 4A was replaced bythe smoothing capacitor C2.

[0246] In the second period T2n, as shown in FIG. 43B, a current I2 nflows increasingly through a closed loop; from the AC source to the ACsource via the inductor L, the load LD, the third diode D3, the fourthswitching element Q4, and the sixth diode D6. The current I2 n flowsindependently of a charge and a discharge of the smoothing capacitorsC1, C2. A simplified equivalent circuit in this period is shown in FIG.4B.

[0247] In the third period T3n, as shown in FIG. 43C, a current I3 nflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the load LD,the parasitic diode of the second switching element Q2, the smoothingcapacitor C1, the smoothing capacitor C2, and the diode D2. The currentI3 n is accompanied by a charging current to the smoothing capacitorsC1, C2. A simplified equivalent circuit in this period is a circuit inwhich the smoothing capacitor C1 of FIG. 4C was replaced by the seriescircuit of the smoothing capacitors C1 and C2.

[0248] As shown in the above explanation, in each of the positive andnegative cycles, a first current supplying mode in which the currentflows through a loop including the AC source, the inductor L, and theload LD is given in the first and second period T1 and T2, and a secondcurrent supplying mode in which the current flows through a loopincluding the inductor L and the load LD but excluding the AC source isgiven in the third period T3. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0249] (Fourteenth Embodiment)

[0250] A power converter in accordance with a fourteenth embodiment ofthe present invention will be explained based on FIGS. 44-47. This powerconverter is designed to convert AC power from an AC source into DCpower and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes four switching elements Q1-Q4, one inductor L, andtwo smoothing capacitors C1, C2. The first switching element Q1 and thesecond switching element Q2 are connected in series with the inductor Land the load LD across the AC source, and the first switching element Q1and the third switching element Q3 are connected in series with theinductor L and the load LD across said AC source. A series circuit of afirst diode D1 and a second diode D2 is connected across a seriescircuit of the second and third switching elements Q2, Q3. A seriescircuit of a first smoothing capacitor C1 and a second smoothingcapacitor C2 is connected across a series circuit of the second andthird switching elements Q2, Q3. A diode bridge D11-D14 is insertedbetween the connection point of the first smoothing capacitor C1 withthe second smoothing capacitor C2 and one terminal of the AC source, andeach input terminal of the diode bridge is connected to the connectionpoint of the first smoothing capacitor C1 with the second smoothingcapacitor C2 and the terminal of the AC source, respectively. The firstswitching element Q1 is connected between output terminals of the diodebridge D11-D14. The one terminal of the AC source is connected with theconnection point of the first diode D1 with the second diode D2. A diodebridge D3-D6 is inserted between the connection point of the first diodeD1 with the second diode D2 and the connection point of the secondswitching element Q2 with the third switching element Q3. The diode D3is connected in series with said diode D4. The diode D5 is connected inseries with the diode D6. The inductor L and the load LD are connectedin series between the connection point of the diode D3 with the diode D4and the connection point of the diode D5 with the diode D6. The fourthswitching element Q4 is connected across a series circuit of the fifthdiode D5 and the sixth diode D6. The second and third switching elementsQ2 and Q3, each defined by FET, have a parasitic diode, respectively,which defines a bypass allowing a reverse current to flow across eachswitching element.

[0251] The control circuit 1 passes a current to the load in onedirection by making the second switching element Q2 turn on and off andkeeping the third switching element Q3 turned off in the meantime, whilemaking the first and fourth switching element Q1, Q4 turn on and offalternately, and also, the control circuit passes the current to theload in the reverse direction by making the third switching element Q3turn on and off and keeping the second switching element Q2 turned offin the meantime, while making the first and fourth switching elementsQ1, Q4 turn on and off alternately.

[0252] The control circuit 1 can always pass the current to both theload and the inductor, improving the harmonic distortion, namely, apower-factor, by controlling the first, second, third, and fourthswitching elements Q1-Q4 to turn on and off in six different patterns.These six patterns are classified into a positive cycle in which threecontinuous patterns are repeated in a positive half cycle of the ACsource, and a negative cycle in which remaining three continuouspatterns are repeated in a negative half cycle of the AC source. Each ofthe positive cycle and the negative cycle is repeated alternately at alow frequency, for example, at 100 Hz. A control system to control theswitching elements Q1-Q4 is shown in FIG. 45.

[0253] In a first pattern in the positive cycle, the first and secondswitching elements Q1, Q2 are turned on. In a second pattern, only thefourth switching element Q4 is turned on. In a third pattern, all theswitching elements Q1-Q4 are turned off. In a first pattern in thenegative cycle, the first and third switching elements Q1, Q3 are turnedon, and in a second pattern, only the fourth switching element Q4 isturned on, and in a third pattern, all the switching elements Q1-Q4 areturned off. Each switching element is turned on and off at a frequencyhigher enough than the frequency of the AC source (50-60 Hz), forexample, dozens several hundred kHz.

[0254]FIGS. 46A-46C show a current which flows through the circuit in afirst period T1p controlled based on the first pattern in the positivecycle, in a second period T2p controlled based on the second pattern inthe positive cycle, and in a third period T3p controlled based on thethird pattern in the positive cycle, respectively. In the first periodT1p, as shown in FIG. 46A, a current I1 p flows increasingly through aclosed loop; from the AC source to the AC source via the diode D13, thefirst switching element Q1, the diode D12, the smoothing capacitor C1,the second switching element Q2, the load LD, and the inductor L. Thecurrent I1 p is accompanied by a discharging current from the smoothingcapacitor C1. A simplified equivalent circuit in this period is shown inFIG. 16A.

[0255] In the second period T2p, as shown in FIG. 46B, a current I2 pflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the fifth diodeD5, the fourth switching element Q4, the diode D4, and the load LD. Thecurrent I2 p flows independently of a charge and a discharge of thesmoothing capacitors C1 and C2. A simplified equivalent circuit in thisperiod is shown in FIG. 16B.

[0256] In the third period T3p, as shown in FIG. 46C, a current I3 pflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the diode D1,the smoothing capacitor C1, the smoothing capacitor C2, the parasiticdiode of the third switching element Q3, and the load LD. The current I3p is accompanied by a charging current to the smoothing capacitors C1and C2. A simplified equivalent circuit in this period is a circuit inwhich the smoothing capacitor C1 of FIG. 16C was replaced by the seriescircuit of the smoothing capacitors C1 and C2.

[0257]FIGS. 47A-47C show a current which flows through the circuit in afirst period T1n controlled based on the first pattern in the negativecycle, in a second period T2n controlled based on the second pattern inthe negative cycle, and in a third period T3n controlled based on thethird pattern in the negative cycle, respectively. In the first periodT1n, as shown in FIG. 47A, a current I1 n flows increasingly through aclosed loop; from the AC source to the AC source via the inductor L, theload LD, the third switching element Q3, the smoothing capacitor C2, thediode D11, the first switching element Q1, and the diode D14. Thecurrent I1 n is accompanied by a discharging current from the smoothingcapacitor C2. A simplified equivalent circuit in this period is acircuit in which the smoothing capacitor C1 of FIG. 16A was replaced bythe smoothing capacitor C2.

[0258] In the second period T2n, as shown in FIG. 47B, a current I2 nflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the load LD,the third diode D3, the fourth switching element Q4, and the diode D6.The current I2 n flows independently of a charge and a discharge of thesmoothing capacitors C1 and C2. A simplified equivalent circuit in thisperiod is shown in FIG. 16B.

[0259] In the third period T3n, as shown in FIG. 47C, a current I3 nflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the load LD,the parasitic diode of the second switching element Q2, the smoothingcapacitor C1, the smoothing capacitor C2, and the diode D2. The currentI3 n is accompanied by a charging current to the smoothing capacitors C1and C2. A simplified equivalent circuit in this period is a circuit inwhich the smoothing capacitor C1 of FIG. 16C was replaced by the seriescircuit of the smoothing capacitors C1 and C2.

[0260] As shown in the above explanation, in each of the positive cycleand negative cycle, a first current supplying mode in which the currentflows through a loop including the AC source, the inductor L, and theload LD is given in the first period T1, and a second current supplyingmode in which the current flows through a loop including the inductor Land the load LD but excluding the AC source is given in the second andthird periods T2, T3. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0261] (Fifteenth Embodiment)

[0262] A power converter in accordance with a fifteenth embodiment ofthe present invention will be explained based on FIGS. 48-51. This powerconverter is designed to convert AC power from an AC source into DCpower and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes four switching elements Q1-Q4, one inductor L, andtwo smoothing capacitors C1, C2.

[0263] The first switching element Q1 and a first smoothing capacitor C1are connected in series with the inductor L and the load LD across theAC source, and the second switching element Q2 and a second smoothingcapacitor C2 are connected in series with the inductor L and the load LDacross the AC source. The first and second switching elements Q1, Q2 areconnected in series, and a series circuit of the first and secondsmoothing capacitors C1, C2 is connected across the series circuit ofthe first and second switching elements Q1, Q2. A first diode D1 and thethird switching element Q3 are connected in series across a seriescircuit of the inductor L and said load LD, and a second diode D2 andthe fourth switching element Q4 are connected in series across theseries circuit of the inductor L and the load LD. A series circuit ofthe third switching element Q3 and the fourth switching element Q4 isconnected across a series circuit of the first and second diodes D1, D2,and the AC source is inserted between the connection point of the firstswitching element Q1 with the second switching element Q2 and theconnection point of the first diode D1 with the second diode D2.

[0264] The control circuit 1 passes the current to the load in onedirection by making the second and third switching elements Q2, Q3 turnoff while making the first and fourth switching elements Q1, Q4 turn onand off alternately. And also, the control circuit passes the current tothe load in the reverse direction by making the first and fourthswitching elements Q1, Q4 turn off while making the second and thirdswitching elements Q2, Q3 turn on and off alternately. The first andsecond switching elements Q1 and Q2, each defined by FET, have aparasitic diode, respectively, which defines a bypass allowing a reversecurrent to flow across each switching element.

[0265] The control circuit 1 can always pass the current to both theload and the inductor, improving the harmonic distortion, namely, apower-factor, by controlling the first, second, third, and fourthswitching elements Q1-Q4 to turn on and off in six different patterns.These six patterns are classified into a positive cycle in which threecontinuous patterns are repeated in a positive half cycle of the ACsource, and a negative cycle in which remaining three continuouspatterns are repeated in a negative half cycle of the AC source. Each ofthe positive cycle and the negative cycle is repeated alternately at alow frequency, for example, at 100 Hz. A control system to control theswitching elements Q1-Q4 is shown in FIG. 49.

[0266] In a first pattern in the positive cycle, only the secondswitching element Q2 is turned on. In a second pattern, only the thirdswitching element Q3 is turned on. In a third pattern, all the switchingelements Q1-Q4 are turned off. In a first pattern in the negative cycle,only the first switching element Q1 is turned on, and in a secondpattern, only the fourth switching element Q4 is turned on, and in athird pattern, all the switching elements Q1-Q4 are turned off. Eachswitching element is turned on and off at a frequency higher enough thanthe frequency of the AC source (50-60 Hz), for example, dozens-severalhundred kHz.

[0267]FIGS. 50A-50C show a current which flows through the circuit in afirst period T1p controlled based on the first pattern in the positivecycle, in a second period T2p controlled based on the second pattern inthe positive cycle, and in a third period T3p controlled based on thethird pattern in the positive cycle, respectively. In the first periodT1p, as shown in FIG. 50A, a current I1 flows increasingly through aclosed loop; from the AC source to the AC source via the secondswitching element Q2, the second smoothing capacitor C2, the load LD,and the inductor L. The current I1 is accompanied by a dischargingcurrent from the second smoothing capacitor C2. A simplified equivalentcircuit in this period is a circuit in which the smoothing capacitor C1of FIG. 8A was replaced by the second smoothing capacitor C2.

[0268] In the second period T2p, as shown in FIG. 50B, a current I2 pflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the first diodeD1, the third switching element Q3, and the load LD. The current I2 pflows independently of a charge and a discharge of the smoothingcapacitors C1 and C2. A simplified equivalent circuit in this period isshown in FIG. 8B.

[0269] In the third period T3p, as shown in FIG. 50C, a current I3 pflows decreasingly through a closed loop; from the AC source to the ACsource via the parasitic diode of the first switching element Q1, thefirst smoothing capacitor C1, the load LD, and the inductor L. Thecurrent I3 p is accompanied by a charging current to the first smoothingcapacitor C1. A simplified equivalent circuit in this period is shown inFIG. 8C.

[0270]FIGS. 51A-51C show a current which flows through the circuit in afirst period T1n controlled based on the first pattern in the negativecycle, in a second period T2n controlled based on the second pattern inthe negative cycle, and in a third period T3n controlled based on thethird pattern in the negative cycle, respectively. In the first periodT1n, as shown in FIG. 51A, a current I1 n flows increasingly through aclosed loop; from the AC source to the AC source via the inductor L, theload LD, the first smoothing capacitor C1, and the first switchingelement Q1. The current I1 n is accompanied by a discharging currentfrom the first smoothing capacitor C1. A simplified equivalent circuitin this period is shown in FIG. 8A.

[0271] In the second period T2n, as shown in FIG. 51B, a current I2 nflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the load LD,the fourth switching element Q4, and the second diode D2. The current I2n flows independently of a charge and a discharge of the smoothingcapacitors C1 and C2. A simplified equivalent circuit in this period isshown in FIG. 8B.

[0272] In the third period T3n, as shown in FIG. 51C, a current I3 nflows decreasingly through a closed loop; from the AC source to the ACsource via the inductor L, the load LD, the second smoothing capacitorC2, and the parasitic diode of the second switching element Q2. Thecurrent I3 p is accompanied by a charging current to the secondsmoothing capacitor C2. A simplified equivalent circuit in this periodis a circuit in which the smoothing capacitor C1 of FIG. 8C was replacedby the second smoothing capacitor C2.

[0273] As shown in the above explanation, in each of the positive cycleand negative cycle, a first current supplying mode in which the currentflows through a loop including the AC source, the inductor L, and theload LD is given in the first and third periods T1, T3, and a secondcurrent supplying mode in which the current flows through a loopincluding the inductor L and the load LD but excluding the AC source isgiven in the second period T2. That is, the control circuit can improvea harmonic distortion (a power-factor) and limit the current to theload, always passing the current to the load and the inductor, byrepeating the first current supplying mode and the second currentsupplying mode alternately.

[0274] (Sixteenth Embodiment)

[0275] A power converter in accordance with a sixteenth embodiment ofthe present invention will be explained based on FIGS. 52-55. This powerconverter is designed to convert AC power from an AC source into DCpower and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes four switching elements Q1-Q4, one inductor L, andtwo smoothing capacitors C1, C2.

[0276] The first switching element Q1 and a first diode D1 are connectedin series with the inductor L and the load LD across the AC source, thesecond switching element Q2 and a second diode D2 are connected inseries with the inductor L and the load LD across the AC source. Thefirst and second switching elements Q1, Q2 are connected in series, anda series circuit of the first and second diodes D1, D2 and a smoothingcapacitor C1 are connected across the series circuit of the first andsecond switching elements Q1, 02. A series circuit of the third andfourth switching elements Q3, Q4 is connected across a series circuit ofthe first and second switching element Q1, Q2. The AC source is insertedbetween the connection point of the first diode D1 with the second diodeD2 and the connection point of the third switching element Q3 with thefourth switching element Q4. The inductor L and the load LD are insertedin series between the connection point of the first switching element Q1with the second switching element Q2 and the connection point of thethird switching element Q3 with the fourth switching element Q4. Thefirst and second switching elements Q1 and Q2, each defined by FET, havea parasitic diode, respectively, which defines a bypass allowing areverse current to flow across each switching element.

[0277] The control circuit 1 passes the current to the load in onedirection by making the second and third switching elements Q2, Q3 turnoff while making the first and fourth switching elements Q1, Q4 turn onand off. And also, the control circuit passes the current to the load inthe reverse direction by making the first and fourth switching elementsQ1, Q4 turn off while making the second and third switching elements Q2,Q3 turn on and off.

[0278] The control circuit 1 can always pass the current to both theload and the inductor, improving the harmonic distortion, namely, apower-factor, by controlling the first, second, third, and fourthswitching elements Q1-Q4 in six different patterns. These six patternsare classified into a positive cycle in which three continuous patternsare repeated in a positive half cycle of the AC source, and a negativecycle in which remaining three continuous patterns are repeated in anegative half cycle of the AC source. Each of the positive cycle and thenegative cycle is repeated alternately at a low frequency, for example,at 100 Hz. FIG. 53 shows a control system to control the switchingelements Q1-Q4.

[0279] In a first pattern in the positive cycle, the first and fourthswitching elements Q1, Q4 are turned on. In a second pattern, only thefirst switching element Q1 is turned on. In a third pattern, all theswitching elements Q1-Q4 are turned off. In a first pattern in thenegative cycle, the second and third switching elements Q2, Q3 areturned on, and in a second pattern, only the second switching element Q2is turned on, and in a third pattern, all the switching elements Q1-Q4are turned off. Each switching element is turned on and off at afrequency higher enough than the frequency of the AC source (50-60 Hz),for example, dozens-several hundred kHz.

[0280]FIGS. 54A-54C show a current which flows through the circuit in afirst period T1p controlled based on the first pattern in the positivecycle, in a second period T2p controlled based on the second pattern inthe positive cycle, and in a third period T3p controlled based on thethird pattern in the positive cycle, respectively. In the first periodT1p, as shown in FIG. 54A, a current I1 p flows increasingly through aclosed loop; from the smoothing capacitor C1 to the smoothing capacitorC1 via the first switching element Q1, the load LD, the inductor L, andthe fourth switching element Q4. The current I1 p is accompanied by adischarging current from the smoothing capacitor C1. A simplifiedequivalent circuit in the first period is shown in FIG. 12A.

[0281] In the second period T2p, as shown in FIG. 54B, a current I2 pflows increasingly through a closed loop; from the AC source to the ACsource via the first diode D1, the first switching element Q1, the loadLD, and the inductor L. The current I2 p flows independently of a chargeand a discharge of the smoothing capacitor C1. A simplified equivalentcircuit in the second period is shown in FIG. 12B.

[0282] In the third period T3p, as shown in FIG. 54C, a current I3 pflows decreasingly through a closed loop; from the AC source to the ACsource via the first diode D1, the smoothing capacitor C1, the parasiticdiode of the second switching element Q2, the load LD, and the inductorL. The current I3 p is accompanied by a charging current to thesmoothing capacitor C1. A simplified equivalent circuit in this periodis shown in FIG. 12C.

[0283]FIGS. 55A-55C show a current which flows through the circuit in afirst period T1n controlled based on the first pattern in the negativecycle, in a second period T2n controlled based on the second pattern inthe negative cycle, and in a third period T3n controlled based on thethird pattern in the negative cycle, respectively. In the first periodT1n, as shown in FIG. 55A, a current I1 n flows increasingly through aclosed loop; from the smoothing capacitor C1 to the smoothing capacitorC1 via the third switching element Q3, the inductor L, the load LD, andthe second switching element Q2. The current I1 n is accompanied by adischarging current from the smoothing capacitor C1. A simplifiedequivalent circuit in the first period is shown in FIG. 12A.

[0284] In the second period T2n, as shown in FIG. 55B, a current I2 nflows increasingly through a closed loop; from the AC source to the ACsource via the inductor L, the load LD, the second switching element Q2,and the second diode D2. The current I2 n flows independently of acharge and a discharge of the smoothing capacitor C1. A simplifiedequivalent circuit in the second period is shown in FIG. 12B.

[0285] In the third period T3n, as shown in FIG. 55C, a current I3 nflows decreasingly through a closed loop; from the AC source to the ACsource via the inductor L, the load LD, the parasitic diode of the firstswitching element Q1, the smoothing capacitor C1, and the second diodeD2. The current I3 n is accompanied by a charging current to thesmoothing capacitor C1. A simplified equivalent circuit in this periodis shown in FIG. 12C.

[0286] As shown in the above explanation, in each of the positive cycleand negative cycle, a first current supplying mode in which the currentflows through a loop including the AC source, the inductor L, and theload LD is given in the second and third periods T2, T3, and a secondcurrent supplying mode in which the current flows through a loopincluding the inductor L and the load LD but excluding the AC source isgiven in the first period T1. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0287] (Seventeenth Embodiment)

[0288] A power converter in accordance with a seventeenth embodiment ofthe present invention will be explained based on FIGS. 56-59. This powerconverter is designed to convert AC power from an AC source into DCpower and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes four switching elements Q1-Q4, one inductor L, andtwo smoothing capacitors C1, C2.

[0289] A first diode D1 and the first switching element Q1 are connectedin series with the inductor L and the load LD across the AC source, andthe second switching element Q2 and a second diode D2 are connected inseries with the inductor L and the load LD across the AC source. Thefirst and second switching elements Q1, Q2 are connected in series, andthe series circuit is connected across a series circuit of the firstdiode D1 and the second diode D2. The first smoothing capacitor C1 andthe third switching element Q3 are connected in series across a seriescircuit of the inductor L and the load LD. The fourth switching elementQ4 and the second smoothing capacitor C2 are connected in series acrossthe series circuit of the inductor L and the load LD. The first andsecond smoothing capacitors C1, C2 are connected in series, and theseries circuit of the first and second smoothing capacitor is connectedacross a series circuit of the third and fourth switching elements Q3,Q4. The AC source is connected between the connection point of the firstdiode D1 with the second diode D2 and the connection point of the firstsmoothing capacitor C1 with the second smoothing capacitor C2. The thirdand fourth switching elements Q3 and Q4, each defined by FET, have aparasitic diode, respectively, which defines a bypass allowing a reversecurrent to flow across each switching element.

[0290] The control circuit 1 passes the current to the load in onedirection by making the second and fourth switching elements Q2, Q4 turnoff while making the first and third switching elements Q1, Q3 turn onand off alternately. And also, the control circuit passes the current tothe load in the reverse direction by making the first and thirdswitching elements Q1, Q3 turn off while making the second and fourthswitching elements Q2, Q4 turn on and off alternately.

[0291] The control circuit 1 can always pass the current to both theload and the inductor, improving the harmonic distortion, namely, apower-factor, by controlling the first, second, third, and fourthswitching elements Q1-Q4 in six different patterns. These six patternsare classified into a positive cycle in which three continuous patternsare repeated in a positive half cycle of the AC source, and a negativecycle in which remaining three continuous patterns are repeated in anegative half cycle of the AC source. Each of the positive cycle and thenegative cycle is repeated alternately at a low frequency, for example,at 100 Hz. A control system to control the switching elements Q1-Q4 isshown in FIG. 57.

[0292] In a first pattern in the positive cycle, only the thirdswitching element Q3 is turned on. In a second pattern, only the firstswitching element Q1 is turned on. In a third pattern, all the switchingelements Q1-Q4 are turned off. In a first pattern in the negative cycle,only the fourth switching element Q4 is turned on, and in a secondpattern, only the second switching element Q2 is turned on, and in athird pattern, all the switching elements Q1-Q4 are turned off. Eachswitching element is turned on and off at a frequency higher enough thanthe frequency of the AC source (50-60 Hz), for example, dozens-severalhundred kHz.

[0293]FIGS. 58A-58C show a current which flows through the circuit in afirst period T1p controlled based on the first pattern in the positivecycle, in a second period T2p controlled based on the second pattern inthe positive cycle, and in a third period T3p controlled based on thethird pattern in the positive cycle, respectively. In the first periodT1p, as shown in FIG. 58A, a current I1 p flows increasingly through aclosed loop; from the smoothing capacitor C1 to the smoothing capacitorC1 via the third switching element Q3, the load LD, and the inductor L.The current I1 p is accompanied by a discharging current from thesmoothing capacitor C1. A simplified equivalent circuit in this periodis shown in FIG. 20A.

[0294] In the second period T2p, as shown in FIG. 58B, a current I2 pflows increasingly through a closed loop; from the AC source to the ACsource via the first diode D1, the first switching element Q1, the loadLD, and the inductor L. The current I2 p flows independently of a chargeand a discharge of the smoothing capacitor C1. A simplified equivalentcircuit in this period is shown in FIG. 20B.

[0295] In the third period T3p, as shown in FIG. 58C, a current I3 pflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the secondsmoothing capacitor C2, the parasitic diode of the fourth switchingelement Q4, and the load LD. The current I3 p is accompanied by acharging current to the second smoothing capacitor C2. A simplifiedequivalent circuit in this period is a circuit in which the smoothingcapacitor C1 of FIG. 20C was replaced by the second smoothing capacitorC2.

[0296]FIGS. 59A-59C show a current which flows through the circuit in afirst period T1n controlled based on the first pattern in the negativecycle, in a second period T2n controlled based on the second pattern inthe negative cycle, and in a third period T3n controlled based on thethird pattern in the negative cycle, respectively. In the first periodT1n, as shown in FIG. 59A, a current I1 n flows increasingly through aclosed loop; from the smoothing capacitor C2 to the smoothing capacitorC2 via the inductor L, the load LD, and the fourth switching element Q4.The current I1 n is accompanied by a discharging current from thesmoothing capacitor C2. A simplified equivalent circuit in this periodis a circuit in which the smoothing capacitor C1 of FIG. 20A wasreplaced by the second smoothing capacitor C2.

[0297] In the second period T2n, as shown in FIG. 59B, a current I2 nflows increasingly through a closed loop; from the AC source to the ACsource via the inductor L, the load LD, the second switching element Q2,and the second diode D2. The current I2 n flows independently of acharge and a discharge of the smoothing capacitors C1, C2. A simplifiedequivalent circuit in this period is shown in FIG. 20B.

[0298] In the third period T3n, as shown in FIG. 59C, a current I3 nflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the load LD,the parasitic diode of the third switching element Q3, and the firstsmoothing capacitor C1. The current I3 n is accompanied by a chargingcurrent to the first smoothing capacitor C1. A simplified equivalentcircuit in this period is shown in FIG. 20C.

[0299] As shown in the above explanation, in each of the positive cycleand negative cycle, a first current supplying mode in which the currentflows through a loop including the AC source, the inductor L, and theload LD is given in the second period T2, and a second current supplyingmode in which the current flows through a loop including the inductor Land the load LD but excluding the AC source is given in the first andthird periods T1, T3. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0300] (Eighteenth Embodiment)

[0301] A power converter in accordance with an eighteenth embodiment ofthe present invention will be explained based on FIGS. 60-63. This powerconverter is designed to convert AC power from an AC source into DCpower and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes four switching elements Q1-Q4, one inductor L, andtwo smoothing capacitors C1, C2.

[0302] A first diode D1 and a first smoothing capacitor C1 are connectedin series with the inductor L and the load LD across the AC source, anda second diode D2 and a second smoothing capacitor C2 are connected inseries with the inductor L and the load LD across the AC source. Thefirst smoothing capacitor D1 and the second diode D2 are connected inseries, and the series circuit of the first and second smoothingcapacitors are connected across a series circuit of the first smoothingcapacitor C1 and the second smoothing capacitor C2. A series circuit ofthe first and second switching elements Q1, Q2 is connected across aseries circuit of the first and second diodes D1, D2. A series circuitof a third diode D3 and the third switching element Q3 is connectedacross a series circuit of the inductor L and the load LD, and a seriescircuit of a fourth diode D4 and a fourth switching element Q4 isconnected across the series circuit of the inductor L and the load LD. Aseries circuit of the third switching element Q3 and the fourthswitching element Q4 is connected across a series circuit of the thirddiode D3 and the fourth diode D4. The AC source is inserted between theconnection point of the first diode D1 with the second diode D2 and theconnection point of the first switching element Q1 with the secondswitching element Q2.

[0303] The control circuit passes the current to the load in onedirection by making the second and third switching elements Q2, Q3 turnoff while making the first and fourth switching elements Q1, Q4 turn onand off alternately. And also, the control circuit passes the current tothe load in the reverse direction by making the first and fourthswitching elements Q1, Q4 turn off while making the second and thirdswitching elements Q2, Q3 turn on and off alternately.

[0304] The control circuit 1 can always pass the current to both theload and the inductor, improving the harmonic distortion, namely, apower-factor, by controlling the first, second, third, and fourthswitching elements Q1-Q4 to turn on and off in six different patterns.These six patterns are classified into a positive cycle in which threecontinuous patterns are repeated in a positive half cycle of the ACsource, and a negative cycle in which remaining three continuouspatterns are repeated in a negative half cycle of the AC source. Each ofthe positive cycle and the negative cycle is repeated alternately at alow frequency, for example, at 100 Hz. FIG. 61 shows a control system tocontrol the switching elements Q1-Q4.

[0305] In a first pattern in the positive cycle, only the secondswitching element Q2 is turned on. In a second pattern, only the thirdswitching element Q3 is turned on. In a third pattern, all the switchingelements Q1-Q4 are turned off. In a first pattern in the negative cycle,only the first switching element Q1 is turned on, and in a secondpattern, only the fourth switching element Q4 is turned on, and in athird pattern, all the switching elements Q1-Q4 are turned off. Eachswitching element is turned on and off at a frequency higher enough thanthe frequency of the AC source (50-60 Hz), for example, dozens-severalhundred kHz.

[0306]FIGS. 62A-62C show a current which flows through the circuit in afirst period T1p controlled based on the first pattern in the positivecycle, in a second period T2p controlled based on the second pattern inthe positive cycle, and in a third period T3p controlled based on thethird pattern in the positive cycle, respectively. In the first periodT1p, as shown in FIG. 62A, a current I1 p flows increasingly through aclosed loop; from the smoothing capacitor C2 to the smoothing capacitorC2 via the load LD, and the inductor L, and the second switching elementQ2. The current I1 p is accompanied by a discharging current from thesmoothing capacitor C2. A simplified equivalent circuit in this periodis a circuit in which the smoothing capacitor C1 of FIG. 24A wasreplaced by the second smoothing capacitor C2.

[0307] In the second period T2p, as shown in FIG. 62B, a current I2 pflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the diode D3,the third switching element Q3, and the load LD. The current I2 p flowsindependently of a charge and a discharge of the smoothing capacitorsC1, C2. A simplified equivalent circuit in this period is shown in FIG.24B.

[0308] In the third period T3p, as shown in FIG. 62C, a current I3 pflows decreasingly through a closed loop; from the AC source to the ACsource via the diode D1, the smoothing capacitor C1, the load LD, andthe inductor L. The current I3 p is accompanied by a charging current tothe smoothing capacitor C1. A simplified equivalent circuit in thisperiod is shown in FIG. 24C.

[0309]FIGS. 63A-63C show a current which flows through the circuit in afirst period T1n controlled based on the first pattern in the negativecycle, in a second period T2n controlled based on the second pattern inthe negative cycle, and in a third period T3n controlled based on thethird pattern in the negative cycle, respectively. In the first periodT1n, as shown in FIG. 63A, a current I1 n flows increasingly through aclosed loop; from the smoothing capacitor C1 to the smoothing capacitorC1 via the first switching element Q1, the inductor L, and the load LD.The current I1 n is accompanied by a discharging current from thesmoothing capacitor C1. A simplified equivalent circuit in this periodis shown in FIG. 24A.

[0310] In the second period T2n, as shown in FIG. 63B, a current I2 nflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the load LD,the fourth switching element Q4, and the diode D4. The current I2 nflows independently of a charge and a discharge of the smoothingcapacitors C1, C2. A simplified equivalent circuit in this period isshown in FIG. 24B.

[0311] In the third period T3n, as shown in FIG. 63C, a current I3 nflows decreasingly through a closed loop; from the AC source to the ACsource via the inductor L, the load LD, the smoothing capacitor C2, andthe diode D2. The current I3 n is accompanied by a charging current tothe smoothing capacitor C2. A simplified equivalent circuit in thisperiod is a circuit in which the smoothing capacitor C1 of FIG. 24C wasreplaced by the smoothing capacitor C2.

[0312] As shown in the above explanation, in each of the positive cycleand negative cycle, a first current supplying mode in which the currentflows through a loop including the AC source, the inductor L, and theload LD is given in the third period T3, and a second current supplyingmode in which the current flows through a loop including the inductor Land the load LD but excluding the AC source is given in the first andsecond periods T1, T2. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0313] (Nineteenth Embodiment)

[0314] A power converter in accordance with a nineteenth embodiment ofthe present invention will be explained based on FIGS. 64-67. This powerconverter is designed to convert AC power from an AC source into DCpower and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes four switching elements Q1-Q4, one inductor L, andtwo smoothing capacitors C1, C2. The second and fourth switchingelements Q2 and Q4, each defined by FET, have a parasitic diode,respectively, which defines a bypass allowing a reverse current to flowacross each switching element.

[0315] A first diode D1, the first switching element Q1, and a seconddiode D2 are connected in series with the inductor L and the load LDacross the AC source, and the first diode D1, the parasitic diode of thesecond switching element Q2, the second diode D2, and a smoothingcapacitor C1 are connected in series with the inductor L and the load LDacross the AC source. And also, a third diode D3, the third switchingelement Q3, and a fourth diode D4 are connected in series with theinductor L and the load LD across the AC source, and the third diode D3,the parasitic diode of the fourth switching element Q4, the smoothingcapacitor C1, and the fourth diode D4 are connected in series with theinductor L and the load LD across the AC source. A series circuit of thefirst switching element Q1, the fourth switching element Q4, theinductor L, and the load LD is connected across the smoothing capacitorC1, and also, a series circuit of the second switching element Q2, thethird switching element Q3, the inductor L, and the load LD is connectedacross the smoothing capacitor C1.

[0316] The control circuit 1 passes the current to the load in onedirection by making the second and third switching elements Q2, Q3 turnoff while making the first and fourth switching elements Q1, Q4 turn onand off with different duty ratio. And also, the control circuit passesthe current to the load in the reverse direction by making the first andfourth switching elements Q1, Q4 turn off while making the second andthird switching elements Q2, Q3 turn on and off with different dutyratio.

[0317] The control circuit 1 can always pass the current to both theload and the inductor, improving the harmonic distortion, namely, apower-factor, by controlling the first, second, third, and fourthswitching elements Q1-Q4 in six different patterns. These six patternsare classified into a positive cycle in which three continuous patternsare repeated in a positive half cycle of the AC source, and a negativecycle in which remaining three continuous patterns are repeated in anegative half cycle of the AC source. Each of the positive cycle and thenegative cycle is repeated alternately at a low frequency, for example,at 100 Hz. A control system to control the switching elements Q1-Q4 isshown in FIG. 65.

[0318] In a first pattern in the positive cycle, the first and fourthswitching elements Q1, Q4 are turned on. In a second pattern, only thefirst switching element Q1 is turned on. In a third pattern, all theswitching elements Q1-Q4 are turned off. In a first pattern in thenegative cycle, the second and third switching elements Q2, Q3 areturned on, and in a second pattern, only the third switching element Q3is turned on, and in a third pattern, all the switching elements Q1-Q4are turned off. Each switching element is turned on and off at afrequency higher enough than the frequency of the AC source (50-60 Hz),for example, dozens-several hundred kHz.

[0319]FIGS. 66A-66C show a current which flows through the circuit in afirst period T1p controlled based on the first pattern in the positivecycle, in a second period T2p controlled based on the second pattern inthe positive cycle, and in a third period T3p controlled based on thethird pattern in the positive cycle, respectively. In the first periodT1p, as shown in FIG. 62A, a current I1 p flows increasingly through aclosed loop; from the smoothing capacitor C1 to the smoothing capacitorC1 via the fourth switching element Q4, the load LD, the inductor L, andthe first switching element Q1. The current I1 is accompanied by adischarging current from the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 12A.

[0320] In the second period T2p, as shown in FIG. 66B, a current I2 pflows increasingly through a closed loop; from the AC source to the ACsource via the first diode D1, the load LD, the inductor L, the firstswitching element Q1, and the second diode D2. The current I2 p flowsindependently of a charge and a discharge of the smoothing capacitor C1.A simplified equivalent circuit in this period is shown in FIG. 12B.

[0321] In the third period T3p, as shown in FIG. 66C, a current I3 pflows decreasingly through a closed loop; from the AC source to the ACsource via the first diode D1, the load LD, the inductor L, theparasitic diode of the second switching element Q2, and the smoothingcapacitor C1, and the second diode D2. The current I3 p is accompaniedby a charging current to the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 12C.

[0322]FIGS. 67A-67C show a current which flows through the circuit in afirst period T1n controlled based on the first pattern in the negativecycle, in a second period T2n controlled based on the second pattern inthe negative cycle, and in a third period T3n controlled based on thethird pattern in the negative cycle, respectively. In the first periodT1n, as shown in FIG. 67A, a current I1 n flows increasingly through aclosed loop; from the smoothing capacitor C1 to the smoothing capacitorC1 via the second switching element Q2, the inductor L, the load LD, andthe third switching element Q3. The current I1 n is accompanied by adischarging current from the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 12A.

[0323] In the second period T2n, as shown in FIG. 67B, a current I2 nflows increasingly through a closed loop; from the AC source to the ACsource via the third diode D3, the inductor L, the load LD, the thirdswitching element Q3, and the fourth diode D4. The current I2 n flowsindependently of a charge and a discharge of the smoothing capacitor C1.A simplified equivalent circuit in this period is shown in FIG. 12B.

[0324] In the third period T3n, as shown in FIG. 67C, a current I3 nflows decreasingly through a closed loop; from the AC source to the ACsource via the third diode D3, the inductor L, the load LD, theparasitic diode of the fourth switching element Q4, the smoothingcapacitor C1, and the fourth diode D4. The current I3 n is accompaniedby a charging current to the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 12C.

[0325] As shown in the above explanation, in each of the positive cycleand negative cycle, a first current supplying mode in which the currentflows through a loop including the AC source, the inductor L, and theload LD is given in the second and third periods T2, T3, and a secondcurrent supplying mode in which the current flows through a loopincluding the inductor L and the load LD but excluding the AC source isgiven in the first period T1. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0326] Although, in this embodiment, it is required to control eachswitching element according to the polarity of the AC source, it ispossible to discriminate the polarity of the AC source, unaffectedly byhigh-frequency voltage, by a simple detection circuit for detecting thevoltage of a connection point X between the first diode D1 and thefourth diode D4, and the voltage of a connection point Y between thesecond diode D2 and the third diode D3, and therefore it is possible tocontrol each of the switching elements; because either the connectionpoint X or the connection point Y is equipotential with thenegative-pole side of the smoothing capacitor C1 according to thepolarity of the AC source and is not affected by the high-frequencyvoltage produced by turning on and off of the switching elements, thatis, electrical potential at the connection point X and Y changes at thesame low frequency as the AC source.

[0327] (Twentieth Embodiment)

[0328] A power converter in accordance with a twentieth embodiment ofthe present invention is shown in FIGS. 68-70. A circuit arrangement ofthe power converter is identical to the nineteenth embodiment, and acontrol system of the control circuit 1 is different from the nineteenthembodiment. The similar parts of these embodiments are identified by thesame reference character. The control circuit 1 can always pass thecurrent to the load and the inductor, improving the harmonic distortion,namely, a power-factor, by controlling the first, second, third, andfourth switching elements Q1-Q4 in six different patterns.

[0329] These six patterns are classified into a positive cycle in whichthree continuous patterns are repeated in a positive half cycle of theAC source, and a negative cycle in which remaining three continuouspatterns are repeated in a negative half cycle of the AC source. Each ofthe positive cycle and the negative cycle is repeated alternately at alow frequency, for example, at 100 Hz.

[0330] In a first pattern in the positive cycle, the first and fourthswitching elements Q1, Q4 are turned on. In a second pattern, only thefourth switching element Q4 is turned on. In a third pattern, all theswitching elements Q1-Q4 are turned off. In a first pattern in thenegative cycle, the second and third switching elements Q2, Q3 areturned on, and in a second pattern, only the second switching element Q2is turned on, and in a third pattern, all the switching elements Q1-Q4are turned off. Each switching element is turned on and off at afrequency higher enough than the frequency of the AC source (50-60 Hz),for example, dozens-several hundred kHz.

[0331]FIGS. 69A-69C show a current which flows through the circuit in afirst period T1p controlled based on the first pattern in the positivecycle, in a second period T2p controlled based on the second pattern inthe positive cycle, and in a third period T3p controlled based on thethird pattern in the positive cycle, respectively. In the first periodT1p, as shown in FIG. 69A, a current I1 p flows increasingly through aclosed loop; from the smoothing capacitor C1 to the smoothing capacitorC1 via the fourth switching element Q4, the load LD, the inductor L, andthe first switching element Q1. The current I1 p is accompanied by adischarging current from the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 24A.

[0332] In the second period T2p, as shown in FIG. 69B, a current I2 pflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the parasiticdiode of the second switching element Q2, the fourth switching elementQ4, and the load LD. The current I2 p flows independently of a chargeand a discharge of the smoothing capacitor C1. A simplified equivalentcircuit in this period is shown in FIG. 24B.

[0333] In the third period T3p, as shown in FIG. 69C, a current I3 pflows decreasingly through a closed loop; from the AC source to the ACsource via the first diode D1, the load LD, the inductor L, theparasitic diode of the second switching element Q2, the smoothingcapacitor C1, and the second diode D2. The current I3 p is accompaniedby a charging current to the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 24C.

[0334]FIGS. 70A-70C show a current which flows through the circuit in afirst period T1n controlled based on the first pattern in the negativecycle, in a second period T2n controlled based on the second pattern inthe negative cycle, and in a third period T3n controlled based on thethird pattern in the negative cycle, respectively. In the first periodT1n, as shown in FIG. 70A, a current I1 n flows increasingly through aclosed loop; from the smoothing capacitor C1 to the smoothing capacitorC1 via the second switching element Q2, the inductor L, the load LD, andthe third switching element Q3. The current I1 n is accompanied by adischarging current from the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 24A.

[0335] In the second period T2n, as shown in FIG. 70B, a current I2 nflows decreasingly by back electromotive force of the inductor L througha closed loop; from the inductor L to the inductor L via the load LD,the parasitic diode of the fourth switching element Q4, and the secondswitching element Q2. The current I2 n flows independently of a chargeand a discharge of the smoothing capacitor C1. A simplified equivalentcircuit in this period is shown in FIG. 24B.

[0336] In the third period T3n, as shown in FIG. 70C, a current I3 nflows decreasingly through a closed loop; from the AC source to the ACsource via the third diode D3, the inductor L, the load LD, theparasitic diode of the fourth switching element Q4, the smoothingcapacitor C1, and the fourth diode D4. The current I3 n is accompaniedby a charging current to the smoothing capacitor C1. A simplifiedequivalent circuit in this period is shown in FIG. 24C.

[0337] As shown in the above explanation, in each of the positive cycleand negative cycle, a first current supplying mode in which the currentflows through a loop including the AC source, the inductor L, and theload LD is given in the third period T3, and a second current supplyingmode in which the current flows through a loop including the inductor Land the load LD but excluding the AC source is given in the first andsecond periods T1, T2. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0338] (Twenty-First Embodiment)

[0339] A power converter in accordance with a twenty-first embodiment ofthe present invention will be explained based on FIGS. 71-73. This powerconverter has a inverter circuit to which a discharge lamp La isconnected as a load LD, and supplies DC power to the input terminals ofthe inverter circuit. In the inverter circuit, four switching elementsQ11-Q14 are connected to form a full bridge, and the discharge lamp Lais connected between the output terminals of the inverter circuit.

[0340] This power converter has a rectifier circuit DB which rectifiesan AC current from an AC source, two switching elements Q1 and Q2, oneinductor L, and one smoothing capacitor C1. The first switching elementQ1 is connected in series with a inductor L and the load LD across therectifier circuit DB, and a first diode D1, the smoothing capacitor C1,and a second diode D2 are connected in series across the inductor L. Aseries circuit of the second switching element Q2, the inductor L, andthe load LD is connected across the smoothing capacitor C1. The firstdiode D1 and the second switching element Q2 are connected in seriesacross the inductor L. The first and second diodes D1, D2 define arectifying device which shunts the current to be supplied to the load LDfrom the inductor L, to the smoothing capacitor.

[0341] The control circuit can improve a power-factor and can limit thecurrent to the load, always passing the current to both the load and theinductor, by controlling the first and second switching elements so thatthey can have both a period in which they are turned on and offalternately and a period in which they are turned off at the same time.

[0342]FIG. 72 shows a control system of the first and second switchingelements Q1, Q2 and the eleventh-fourteenth switching elements Q11-Q14.The first and second switching elements Q1, Q2 are controlled in threepatterns. In a first pattern, the second switching element Q2 is turnedon. In a second pattern, the first switching element Q1 is turned on. Ina third pattern, both of the switching elements Q1, Q2 are turned off.An alternating power of low frequency can be supplied to the dischargelamp La by controlling the switching elements Q11-Q14, which form theinverter circuit as the load, in synchronization with the AC source,while these three patterns are repeated. Each of the switching elementsQ1, Q2 are turned on and off at a frequency higher enough than thefrequency of the AC source (50-60 Hz), for example, dozens-severalhundred kHz. A low-pass filter which prevents the high frequencycomponent produced by turning on and off actions of the switchingelements Q1 and Q2 from being superposed on the AC power is insertedbetween the AC source and the rectifier circuit DB.

[0343]FIGS. 73A-73C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 73A, the current flows increasingly througha closed loop; from the smoothing capacitor C1 to the smoothingcapacitor C1 via the second switching element Q2, the inductor L, andthe load LD, and the current to the load LD is limited by the inductorL. The current is accompanied by a discharging current from thesmoothing capacitor C1.

[0344] In the second period T2, as shown in FIG. 73B, the current flowsthrough a closed loop; from the AC source to the AC source via therectifier circuit DB, the first switching element Q1, the inductor L,the load LD, and the rectifier circuit DB. In this period, whether thecurrent flowing through the inductor L will increase or decrease,namely, whether energy will be stored in the inductor L or dischargedfrom the inductor L is changed according to the output voltage of therectifier circuit DB.

[0345] In the third period T3, as shown in FIG. 73C, the current flowsby stored energy of the inductor L through a closed loop; from theinductor L to the inductor L via the load LD, and the second diode D2.

[0346] As shown in the above explanation, a first current supplying modein which the current flows through a loop including the AC source, theinductor L, and the load LD is given in the second and third periods T2,T3, and a second current supplying mode in which the current flowsthrough a loop including the inductor L and the load LD but excludingthe AC source is given in the first period T1. That is, the controlcircuit can improve a harmonic distortion (a power-factor) and limit thecurrent to the load, always passing the current to the load and theinductor, by repeating the first current supplying mode and the secondcurrent supplying mode alternately.

[0347] In this embodiment, the first switching element Q1, the inductorL, and the second diode D2 form a step-down chopper which regards therectifier circuit as a power supply and supplies power to the load LD.As the switching element, a well-known thing such as MOSFET, a bipolartransistor, and IGBT may be used. Also, the second switching element Q2,the inductor L, and the second diode D2 form a step-down chopper whichregards the smoothing capacitor C1 as a power supply and supplies powerto the load LD.

[0348] In the period T3 shown in FIG. 73C, if a surplus of the storedenergy of the Inductor L is generated and the voltage across theinductor L becomes higher than the voltage across the smoothingcapacitor C1, the first diode D1 will conduct a current, and the storedenergy of the inductor L will be used for a charge of the smoothingcapacitor C1 through a closed loop; form the inductor L to the inductorL via the first diode D1, the smoothing capacitor C1, and the seconddiode D2. For example, in a crest of the voltage wave where the outputvoltage of the rectifier circuit DB is near the peak, the surplus of thestored energy of the inductor L is generated, and the smoothingcapacitor C1 is charged. That is, it is prevented that the supplyvoltage to the load becomes superfluous, because the first diode D1 isconnected in series with the inductor L and the smoothing capacitor C1across the rectifier circuit DB and the current is shunted to thesmoothing capacitor C1 through the first diode D1 near the peak voltageof the AC source.

[0349] On the other hand, in the trough of the voltage wave where theoutput voltage of the rectifier circuit DB is near OV, the stored energyof the inductor L is supplied only to the load LD. In the state of FIG.73B, too, the first diode D1 may conduct a current, and the smoothingcapacitor C1 may be charged, depending on a relation between theelectrical potential of the connection point of the inductor L with theload LD and the electrical potential of the positive pole of thesmoothing capacitor C1.

[0350] Moreover, if a surplus of the input energy is generated and thevoltage across the smoothing capacitor C1 rises, the first diode D1 doesnot conduct the current in the state of FIG. 73C, then the length inwhich the current flows to the load LD becomes longer. Therefore, theoutput energy to the load LD increases. On the other hand, if a shortageof the input energy is generated and the voltage across the smoothingcapacitor C1 falls, the length in which the charging current flows tothe smoothing capacitor C1 becomes longer, therefore, the output energyto the load LD decreases. In this way, because the output energy to theload LD will increase or decrease according to the increasing ordecreasing of the input energy, a balance between the input energy andthe output energy is automatically adjusted so that the balance isneither too much nor too little. Therefore, it is easy to control theswitching elements Q1, Q2, and it becomes possible to control them by asimple control circuit.

[0351] (Twenty-Second Embodiment)

[0352] A power converter in accordance with a twenty-second embodimentof the present invention is shown in FIGS. 74 and 75. A circuitarrangement of the power converter is identical to the twenty-firstembodiment except that the inductor L includes a primary winding n1 anda secondary winding n2. The similar parts of these embodiments areidentified by the same reference character.

[0353] In this power converter, the first switching element Q1 isconnected in series with the primary winding n1 of the inductor L andthe load LD across the rectifier circuit DB. The second switchingelement Q2, the inductor L, and the load LD are connected in seriesacross the smoothing capacitor C1. The secondary winding n2 and a firstdiode D1 are connected across the smoothing capacitor C1. The load LDand a second diode D2 are connected in series across the primary windingn1. The polarity of the secondary winding n2 of the inductor L is set upas shown in a FIG. 74.

[0354] The control circuit controls the first and second switchingelements Q1, Q2 so that they can repeat three patterns including aperiod in which they are turned on and off alternately and a period inwhich they are turned off at the same time.

[0355] In a first pattern, the second switching element Q2 is turned on,and as shown in FIG. 75A, a current flows in a closed loop; from thesmoothing capacitor C1 to the smoothing capacitor C1 via the secondswitching element Q2, the primary winding n1 of the inductor L, and theload LD. In this period, a current flowing through the primary windingn1 of the inductor L increases with time, and energy is stored in theinductor L, and the inductor operates as a current-limiting element tothe load LD. Although a voltage is induced in the secondary winding n2in this period, a charging current to the smoothing capacitor C1 doesnot flow, because the inductor is set up so that the induced voltagewill be lower than the voltage across the smoothing capacitor C1.

[0356] In the second period, the first switching element Q1 is turnedon, and a current flows in a closed loop as shown in FIG. 75B; from theAC source to the AC source via the rectifier circuit DB, the firstswitching element Q1, the primary winding n1 of the inductor L, theload, and the rectifier circuit DB. In this period, too, although avoltage is induced in the secondary winding n2 of the inductor L, acharging current to the smoothing capacitor C1 does not flow, becausethe inductor is set up so that the induced voltage will be lower thanthe voltage across the smoothing capacitor C1.

[0357] In the third period, both the first and second switching elementsQ1, Q2 are turned off, and the stored energy of the inductor L isdischarged through two closed loops; one is from the primary winding n1of the inductor L to the primary winding n1 via the load LD, and thesecond diode D2; and the other is from the secondary winding n2 of theinductor L to the secondary winding n2 via the smoothing capacitor C1,and the first diode D1.

[0358] If the voltage across the secondary winding n2 becomes higherthan the voltage across the smoothing capacitor C1, the charging currentflows to the smoothing capacitor C1 from the secondary winding n2. Atthat time, the first diode conducts the current, and the stored energyof the inductor L is used for the charge of the smoothing capacitor C1.In this way, in this arrangement of this embodiment, the voltage appliedto the smoothing capacitor C1 can be set up appropriately by a turnratio of the primary winding n1 and the secondary winding n2, andfreedom of a circuit design can be raised.

[0359] For example, if the turn ratio of the primary winding n1 and thesecondary winding n2 is n, the voltage of the smoothing capacitor isVC1, the output voltage of the rectifier circuit is VE, and the absolutevalue of the load voltage is VLa, then the voltage Vn2 across thesecondary winding n2 is expressed as follows:

[0360] Vn2a=−n(VC1−VLa); In the period of FIG. 75A

[0361] Vn2b=−n(VE−VLa); In the period of FIG. 75B

[0362] Vn2c=n·VLa; In the period of FIG. 75C

[0363] For VE≧0, then Vn2c≧Vn2c.

[0364] Therefore, if the turn ratio n is selected so that VC1=Vn2c, thecharging current can be set up so that it will flow to the smoothingcapacitor C1 from the secondary winding n2 only when in FIG. 75C.

[0365] (Twenty-Third Embodiment)

[0366] A power converter in accordance with a twenty-third embodiment ofthe present invention will be explained based on FIGS. 76-78. This powerconverter has a inverter circuit to which a discharge lamp La isconnected as a load LD, and supplies DC power to the input terminals ofthe inverter circuit. In the inverter circuit, four switching elementsQ11-Q14 are connected to form a full bridge, and the discharge lamp Lais connected between the output terminals of the inverter circuit.

[0367] This power converter has a rectifier circuit DB which rectifiesan AC current from an AC source, two switching elements Q1 and Q2, oneinductor L, and one smoothing capacitor C1. The first switching elementQ1 is connected in series with the inductor L, the load LD, thesmoothing capacitor C1, and the second switching element Q2 across therectifier circuit DB. And the first switching element Q1 is connected inseries with the inductor L, the load LD, and a first diode D1 across therectifier circuit DB. A series circuit of a second diode D2, thesmoothing capacitor C1, and a third diode D3 is inserted across theinductor L, and the second diode D2 and the third diode D3 form a pathto pass a charging current to the smoothing capacitor from the inductor.

[0368] the control circuit can improve a power-factor and can limit thecurrent to the load, always passing the current to the load and theinductor, by controlling the first and second switching elements so thatthey can have both a period in which they are turned on and offalternately and a period in which they are turned off at the same time.

[0369]FIG. 77 shows a control system of the first and second switchingelements Q1, Q2 and the eleventh-fourteenth switching elements Q11-Q14.The first and second switching elements Q1, Q2 are controlled in threepatterns. In a first pattern, the first and second switching elementsQ1, Q2 are turned on. In a second pattern, the first switching elementQ1 is turned on. In a third pattern, both of the switching elements Q1and Q2 are turned off. An alternating power of low frequency is suppliedto the discharge lamp La by controlling the switching elements Q11-Q14,which form the inverter circuit as the load, in synchronization with theAC source, while these three patterns are repeated. Each of theswitching elements Q1, Q2 are turned on and off at a frequency higherenough than the frequency of the AC source (50-60 Hz), for example,dozens-several hundred kHz. A low-pass filter which prevents the highfrequency component produced by turning on and off actions of theswitching elements Q1 and Q2 from being superposed on the AC power isinserted between the AC source and the rectifier circuit DB.

[0370]FIGS. 78A-78C show a current which flows through the circuit in afirst period T1 controlled based on the first pattern, in a secondperiod T2 controlled based on the second pattern, and in a third periodT3 controlled based on the third pattern, respectively. In the firstperiod T1, as shown in FIG. 78A, the current flows through a closedloop; from the smoothing capacitor C1 to the smoothing capacitor C1 viathe second switching element Q2, the rectifier circuit DB, the ACsource, the rectifier circuit DB, the first switching element Q1, theinductor L, and the load LD. In this period, a current flowing throughthe inductor L increases with time, and energy is stored in the inductorL, and the inductor operates as a current-limiting element to the loadLD. By this operation, a voltage adding the voltage across the smoothingcapacitor C1 to the output voltage of the rectifier circuit is appliedto a series circuit of the inductor L and the load LD. In the secondperiod T2, as shown in FIG. 78B, the current flows through a closedloop; from the AC source to the AC source via the rectifier circuit DB,the first switching element Q1, the inductor L, the load LD, the firstdiode D1, and the rectifier circuit DB. In this period, only the outputvoltage of the rectifier circuit DB is applied to the series circuit ofthe inductor L and the load LD, the energy of the inductor L isdischarged.

[0371] In the third period T3, as shown in FIG. 78C, the stored energyof the inductor L is discharged through two closed loops; one is fromthe inductor L to the inductor L via the load LD, and the third diodeD3; and the other is from the inductor L to the inductor L via thesecond diode D2, the smoothing capacitor C1, and the third diode D3. Ifthe voltage across the inductor L becomes higher than the voltage acrossthe smoothing capacitor C1, the charging current flows to the smoothingcapacitor C1 from the secondary winding n2. At that time, the seconddiode conducts the current, and the stored energy of the inductor L isused for the charge of the smoothing capacitor C1.

[0372] As shown in the above explanation, a first current supplying modein which the current flows through a loop including the AC source, theinductor L, and the load LD is given in the first and second periods T1,T2, and a second current supplying mode in which the current flowsthrough a loop including the inductor L and the load LD but excludingthe AC source is given in the third period T3. That is, the controlcircuit can improve a harmonic distortion (a power-factor) and limit thecurrent to the load, always passing the current to the load and theinductor, by repeating the first current supplying mode and the secondcurrent supplying mode alternately.

[0373] (Twenty-Fourth Embodiment)

[0374] A power converter in accordance with a twenty-fourth embodimentof the present invention will be explained based on FIGS. 79-82. Thispower converter is designed to convert AC power from an AC source intoDC power and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes four switching elements Q1-Q4, one inductor L, andone smoothing capacitor C1. The first and second switching elements Q1and Q2, each defined by FET, have a parasitic diode, respectively, whichdefines a bypass allowing a reverse current to flow across eachswitching element.

[0375] A first diode D1, the first switching element Q1, the inductor L,and a load LD are connected in series across the AC source. And, theload LD, the inductor L, the second switching element Q2, and a seconddiode D2 are connected in series across the AC source. The thirdswitching element Q3 and the bypass of the second switching element Q2are connected in series across a series circuit of the inductor L andthe load LD. And, a third diode D3, the smoothing capacitor C1, and thebypass of the second switching element Q2 are connected in series acrossthe inductor L. The bypass of the first switching element Q1 and thefourth switching element Q4 are connected in series across the seriescircuit of the inductor L and the load LD. The bypass of the firstswitching element Q1, the smoothing capacitor C1, and a fourth diode D4are connected in series across the inductor L. And, the third diode D3and the fourth diode D4 form a path to pass a charging current to thesmoothing capacitor from the inductor.

[0376] The control circuit passes the current to the load in onedirection by controlling the first and third switching elements Q1 andQ3 so that both switching elements will repeat three patterns comprisinga period in which both switching elements are turned on at the sametime, and periods in which either of them is turned on, while making thesecond and fourth switching element Q2 and Q4 turn off. And the controlcircuit passes the current to the load in the reverse direction bycontrolling the second and fourth switching element Q2 and Q4 so thatboth switching elements will repeat three patterns comprising a periodin which both switching elements are turned on at the same time, andperiods in which either of them is turned on, while making the first andthird switching elements Q1 and Q3 turn off.

[0377] The control circuit 1 can always pass the current to both theload and the inductor, improving the harmonic distortion, namely, apower-factor, by controlling the first, second, third, and fourthswitching elements Q1-Q4 in six different patterns. These six patternsare classified into a positive cycle in which three continuous patternsare repeated in a positive half cycle of the AC source, and a negativecycle in which remaining three continuous patterns are repeated in anegative half cycle of the AC source. Each of the positive cycle and thenegative cycle is repeated alternately at a low frequency, for example,at 100 Hz. FIG. 80 shows a control system to control the switchingelements Q1-Q4.

[0378] In a first pattern in the positive cycle, the first and thirdswitching elements Q1, Q3 are turned on. In a second pattern, only thefirst switching element Q1 is turned on. In a third pattern, only thethird switching element Q2 is turned on. In a first pattern in thenegative cycle, the second and fourth switching elements Q2, Q4 areturned on, and in a second pattern, only the second switching element Q2is turned on, and in a third pattern, only the fourth element Q4 isturned on. Each switching element is turned on and off at a frequencyhigher enough than the frequency of the AC source (50-60 Hz), forexample, dozens-several hundred kHz.

[0379]FIGS. 81A-81C show a current which flows through the circuit in afirst period T1p controlled based on the first pattern in the positivecycle, in a second period T2p controlled based on the second pattern inthe positive cycle, and in a third period T3p controlled based on thethird pattern in the positive cycle, respectively. In the first periodT1p, as shown in FIG. 81A, the current flows through a closed loop; fromthe smoothing capacitor C1 to the smoothing capacitor C1 via the firstswitching element Q1, the inductor L, the load LD, and the thirdswitching element Q3. In this period, a current flowing through theinductor L increases with time, and energy is stored in the inductor L,and the inductor L operates as a current-limiting element to the loadLD.

[0380] In the second period T2p, as shown in FIG. 81B, the current flowsthrough a closed loop; from the AC source to the AC source via the firstdiode D1, the switching element Q1, the inductor L, and the load LD. Inthis period, whether the current flowing through the inductor L willincrease or decrease, namely, whether energy will be stored in theinductor L or discharged from the inductor L is decided according to themagnitude relation between the voltage across the AC source and thevoltage across the inductor L.

[0381] In the third period T3p, as shown in FIG. 81C, the stored energyof the inductor L is discharged through two closed loops; one is fromthe inductor L to the inductor L via the load LD, the third switchingelement Q3, and the bypass of the second switching element Q2; and theother is from the inductor L to the inductor L via the third diode D3,the smoothing capacitor C1, and the bypass of the second switchingelement Q2. The closed loop including the smoothing capacitor C1 will beformed when the voltage across the inductor L becomes higher than thevoltage of the smoothing capacitor C1 by the stored energy of theinductor L. The ON length of the first switching element Q1, the size ofthe inductor L, etc., are designed so that the voltage generated acrossthe inductor L in the period of FIG. 81C will be higher than the peakvoltage of the AC source. So, in this period, the smoothing capacitor C1is charged by the energy of the inductor L, not by the AC source.

[0382]FIGS. 82A-82C show a current which flows through the circuit in afirst period T1n controlled based on the first pattern in the negativecycle, in a second period T2n controlled based on the second pattern inthe negative cycle, and in a third period T3n controlled based on thethird pattern in the negative cycle, respectively. In the first periodT1n, as shown in FIG. 82A, a current flows through a closed loop; fromthe smoothing capacitor C1 to the smoothing capacitor C1 via the fourthswitching element Q4, the load LD, the inductor L, and the secondswitching element Q2. In this period, a current flowing through theinductor L increases with time, and energy is stored in the inductor L.

[0383] In the second period T2n, as shown in FIG. 82B, a current flowsthrough a closed loop; from the AC source to the AC source via the loadLD, the inductor L, the second switching element Q2, and the seconddiode D2. In this period, whether the current flowing through theinductor L will increase or decrease, namely, whether energy will bestored in the inductor L or discharged from the inductor L is decidedaccording to the magnitude relation between the voltage across the ACsource and the voltage across the inductor L.

[0384] In the third period T3n, as shown in FIG. 82C, the stored energyof the inductor L is discharged through two closed loops; one is fromthe inductor L to the inductor L via the bypass of the first switchingelement Q1, the fourth switching element Q4, and the load LD; and theother is from the inductor L to the inductor L via the bypass of thefirst switching element Q1, the smoothing capacitor C1, and the fourthdiode D4. The closed loop including the smoothing capacitor C1 will beformed when the voltage across the inductor L becomes higher than thevoltage of the smoothing capacitor C1 by the stored energy of theinductor L. The ON length of the second switching element Q2, the sizeof the inductor L, etc., are designed so that the voltage generatedacross the inductor L in the period of FIG. 82C will be higher than thepeak voltage of the AC source. So, in this period, the smoothingcapacitor C1 is charged by the energy of the inductor L, not by the ACsource.

[0385] As shown in the above explanation, in each of the positive andnegative cycles, a first current supplying mode in which the currentflows through a loop including the AC source, the inductor L, and theload LD is given in the second period T2, and a second current supplyingmode in which the current flows through a loop including the inductor Land the load LD but excluding the AC source is given in the first andthird periods T1, T3. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0386] (Twenty-Fifth Embodiment)

[0387] A power converter in accordance with a twenty-fifth embodiment ofthe present invention will be explained based on FIGS. 83-85. This powerconverter is designed to convert AC power from an AC source into DCpower and subsequently convert the DC power to AC power in order tosupply an alternating current of rectangular wave of low frequency, forexample, at 100 Hz, to a load such as a discharge lamp. This powerconverter includes four switching elements Q1-Q4, one inductor L, andone smoothing capacitor C1. The first and second switching elements Q1and Q2, each defined by FET, have a parasitic diode, respectively, whichdefines a bypass allowing a reverse current to flow across eachswitching element. The inductor L has a primary winding n1 and asecondary winding n2.

[0388] A first diode D1, the first switching element Q1, the primarywinding n1 of the inductor L, and the load LD are connected in seriesacross the AC source, and also, the load LD, the primary winding n1, thesecond switching element Q2, and a second diode D2 are connected inseries across the AC source. The third switching element Q3 and thebypass of the second switching element Q2 are connected in series acrossa series circuit of the primary winding n1 and the load LD. The bypassof the first switching element Q1 and the fourth switching element Q4are connected in series across the series circuit of the primary windingn1 and the load LD. A series circuit of a third diode D3, the smoothingcapacitor C1, and a fourth diode D4 is connected across the secondarywinding n2. A series circuit of a fifth diode D5, the smoothingcapacitor C1, and a sixth diode D6 is also connected across thesecondary winding n2. A series circuit of the first switching elementQ1, the primary winding n1, the load LD, the third switching element Q3is inserted across the smoothing capacitor C1. A series circuit of thefourth switching element Q4, the load LD, the primary winding n1, thesecond switching element Q2 is also inserted across the smoothingcapacitor C1. The diodes D3-D6 form a path to pass a charging current tothe smoothing capacitor from the inductor.

[0389] The control circuit passes the current to the load in onedirection by controlling the first and third switching elements Q1, Q3so that both switching elements will repeat three patterns comprising aperiod in which both switching elements are turned on at the same time,and periods in which either of them is turned on, while making thesecond and fourth switching element Q2 and Q4 turn off. And the controlcircuit passes the current to the load in the reverse direction bycontrolling the second and fourth switching elements Q2 and Q4 so thatboth switching elements will repeat three patterns comprising a periodin which both switching elements are turned on at the same time, andperiods in which either of them is turned on, while making the first andthird switching elements Q1 and Q3 turn off.

[0390] The control circuit can always pass the current to both the loadand the inductor, improving the harmonic distortion, namely, apower-factor, by controlling the first, second, third, and fourthswitching elements Q1-Q4 in six different on/off patterns. These sixpatterns are classified into a positive cycle in which three continuouspatterns are repeated in a positive half cycle of the AC source, and anegative cycle in which remaining three continuous patterns are repeatedin a negative half cycle of the AC source. Each of the positive cycleand the negative cycle is repeated alternately at a low frequency, forexample, at 100 Hz. Each switching element is turned on and off at afrequency higher enough than the frequency of the AC source (50-60 Hz),for example, dozens-several hundred kHz.

[0391]FIGS. 84A-84C show a control system in a first pattern of thepositive cycle, in a second pattern of the positive cycle, and in athird pattern of the positive cycle, respectively. As shown in FIG. 84A,in a period of the first pattern, the first and third switching elementsQ1, Q3 are turned on, and a current flows through a closed loop; fromthe smoothing capacitor C1 to the smoothing capacitor C1 via the firstswitching element Q1, the primary winding n1 of the inductor L, the loadLD, and the third switching element Q3. In this period, the currentflowing through the primary winding n1 of the inductor L increases withtime, and energy is stored in the inductor L, and the inductor operatesas a current-limiting element to the load LD.

[0392] As shown in FIG. 84B, in a period of the second pattern, only thefirst switching element Q1 is turned on, and a current flows through aclosed loop; from the AC source to the AC source via the first diode D1,the first switching element Q1, the primary winding n1 of the inductorL, and the load LD. In this period, whether the current flowing throughthe primary winding n1 will increase or decrease, namely, whether energywill be stored in the inductor L or discharged from the inductor L isdecided according to the magnitude relation between the voltage acrossthe AC source and the voltage across the primary winding n1.

[0393] As shown in FIG. 84C, in a period of the third pattern, only thethird switching element Q3 is turned on, and the stored energy of theinductor L is discharged through two closed loops; one is from theprimary winding n1 of the inductor L to the primary winding n1 via theload LD, and the third switching element Q3, and the bypass of thesecond switching element Q2; and the other is from the secondary windingn2 of the inductor L to the secondary winding n2 via the third diode D3,the smoothing capacitor C1, and the fourth diode D4. The closed loopincluding the smoothing capacitor C1 will be formed only when thevoltage across the secondary winding n2 becomes higher than the voltageof the smoothing capacitor C1 by the stored energy of the inductor L.

[0394]FIGS. 85A-85C show a control system in a first pattern of thenegative cycle, in a second pattern of the negative cycle, and in athird pattern of the negative cycle, respectively. As shown in FIG. 85A,in a period of the first pattern, the second and fourth switchingelements Q2, Q4 are turned on, and a current flows through a closedloop; from the smoothing capacitor C1 to the smoothing capacitor C1 viathe fourth switching element Q4, the load LD, the primary winding n1 ofthe inductor L, and the second switching element Q2. In this period, thecurrent flowing through the primary winding n1 of the inductor Lincreases with time, and energy is stored in the inductor L, and theinductor L operates as a current-limiting element to the load LD.

[0395] As shown in FIG. 85B, in a period of the second pattern, only thesecond switching element Q2 is turned on, and a current flows through aclosed loop; from the AC source to the AC source via the load LD, theprimary winding n1 of the inductor L, the second switching element Q2,and the second diode D2. In this period, whether the current flowingthrough the primary winding n1 will increase or decrease, namely,whether energy will be stored in the inductor L or discharged from theinductor L is decided according to the magnitude relation between thevoltage across the AC source and the voltage across the primary windingn1.

[0396] As shown in FIG. 85C, in a period of the third pattern, only thefourth switching element Q4 is turned on, and the stored energy of theinductor L is discharged through two closed loops; one is from theprimary winding n1 of the inductor L to the primary winding n1 via thebypass of the first switching element Q1, the fourth switching elementQ4, and the load LD; and the other is from the secondary winding n2 ofthe inductor L to the secondary winding n2 via the fifth diode D5, thesmoothing capacitor C1, and the sixth diode D6. The closed loopincluding the smoothing capacitor C1 will be formed only when thevoltage across the secondary winding n2 becomes higher than the voltageof the smoothing capacitor C1 by the stored energy of the inductor L.

[0397] As shown in the above explanation, in each of the positive andnegative cycles, a first current supplying mode in which the currentflows through a loop including the AC source, the inductor L, and theload LD is given in the second period T2, and a second current supplyingmode in which the current flows through a loop including the inductor Land the load LD but excluding the AC source is given in the first andthird periods T1, T3. That is, the control circuit can improve aharmonic distortion (a power-factor) and limit the current to the load,always passing the current to the load and the inductor, by repeatingthe first current supplying mode and the second current supplying modealternately.

[0398] In this embodiment, the voltage which is applied to the smoothingcapacitor C1 can be set to a desired value by selecting the turn ratioof the primary winding n1 and the secondary winding n2 appropriately,because the inductor L has the primary winding n1 and the secondarywinding n2, like the twenty-second embodiment, whereby freedom of acircuit design can be raised.

[0399] This applicant is based on and claims the priority of JapanesePatent Application No. 2002-086276, filed on Mar. 26, 2002 and JapanesePatent Application No. 2002-086308, filed on Mar. 26, 2002, the entirecontents of which are expressly incorporated by reference herein.

1. A power converter for providing an electric power from an AC sourceto a load, said power converter comprising: a plurality of switchingelements which turns on and off repetitively to interrupt an inputcurrent from said AC source to provide an output current to said load;an inductor provided in a path of said input current from said AC sourceto said load; a smoothing capacitor which smoothens said input currentto said load; a control circuit for controlling said switching elementsto turn on and off; said inductor and said load being connected inseries across said AC source; said inductor and said load beingconnected in series across said smoothing capacitor; said controlcircuit controlling the plurality of said switching elements to turn onand off in different patterns to give a first current supplying mode anda second current supplying mode; said first current supplying modesupplying said input current from said AC source to a closed loopincluding said inductor and said load, during which the current fromsaid AC source is fed directly to said load, said second currentsupplying mode supplying said output current to said load in a closedloop including said inductor and said load but excluding said AC source,during which energy stored in said inductor supplying a current to saidload, said control circuit repeating said first current supplying modeand said second current supplying mode alternately during each halfcycle of the AC current being supplied from said AC source, therebyconstantly passing the current to said inductor and said load.
 2. Thepower converter as set forth in claim 1, wherein said control circuitcontrols the plurality of said switching elements in three differentpatterns to continuously repeat a first pattern, a second pattern, and athird pattern in this order, one of said three patterns defining one ofsaid first current supplying mode and said second current supplyingmode, the remaining two patterns defining the other of said firstcurrent supplying mode and said second current supplying mode, thevoltage applied to said inductor decreasing in accordance with aprogress from said first pattern to said third pattern.
 3. The powerconverter as set forth in claim 2, wherein said first pattern allowssaid smoothing capacitor to pass a discharge current through saidinductor, said second pattern keeping said smoothing capacitor free fromthe current flowing through said inductor, and said third patternallowing said smoothing capacitor to be charged by the current flowingthrough said inductor.
 4. The power converter as set forth in claim 1,including: a rectifier circuit DB which rectifies the AC current fromsaid AC source to give a DC voltage; said switching elements comprisinga first switching element Q1, a second switching element Q2, and a thirdswitching element Q3, said first switching element Q1, said secondswitching element Q2, and said third switching element Q3 beingconnected in series with said inductor, said load, a first diode D1, andsaid smoothing capacitor C1 across said rectifier circuit DB, a seconddiode D2 being connected across a series circuit of said smoothingcapacitor C1 and said third switching element Q3, said second diode D2being connected in series with a third diode D3 across a series circuitof said inductor L, said first diode D1, said load LD, and said secondswitching element Q2, a fourth diode D4 being connected in series withsaid smoothing capacitor across said second switching element Q2.
 5. Thepower converter as set forth in claim 1, including: a rectifier circuitDB which rectifies the AC current from said AC source to give a DCvoltage; said switching elements comprising a first switching elementQ1, a second switching element Q2, a third switching element Q3, afourth switching element Q4, and a fifth switching element Q5, each ofsaid second switching element Q2 and said fourth switching element Q4having a bypass allowing a reverse current to flow across each switchingelement, said first switching element Q1 and said second switchingelement Q2 being connected in series with a first diode D1 across saidrectifier circuit DB, said first diode D1 being inserted between a highvoltage side of said rectifier DB and said first switching element Q1,said first diode D1 having its cathode connected to said first switchingelement Q1, said third switching element Q3 and said fourth switchingelement Q4 being connected in series with a second diode D2 across saidrectifier circuit DB, said second diode D2 being inserted between a highvoltage side of said rectifier DB and said third switching element Q3,said second diode D2 having its cathode connected to said thirdswitching element Q3, said second switching element Q2 and said fourthswitching element Q4 being connected through a common third diode D3 toa low voltage side of said rectifier DB, said inductor L being connectedin series with said load LD between the connection point of said firstswitching element Q1 with said second switching element Q2 and theconnection point of said third switching element Q3 with said fourthswitching element Q4, said fifth switching element Q5 being connected inseries with said first diode D1, said first switching element Q1, saidinductor L, said load LD, said fourth switching element Q4, and saidsmoothing capacitor C1 across said rectifier DB, said fifth switchingelement Q5 being connected in series with said second diode D2, saidthird switching element Q3, said load LD, said inductor L, said secondswitching element Q2, and said smoothing capacitor C1 across saidrectifier DB, a fourth diode D4 being connected in series with thebypass of said second switching element Q2, said inductor L, and saidload LD across said smoothing capacitor C1, and a fifth diode D5 beingconnected in series with the bypass of said fourth switching element Q4,said load LD, and said inductor L across said smoothing capacitor C1. 6.The power converter as set forth in claim 1, including: a rectifiercircuit DB which rectifies the AC current from said AC source to give aDC voltage; said switching elements comprising a first switching elementQ1, a second switching element Q2, a third switching element Q3, afourth switching element Q4, and a fifth switching element Q5, saidfirst switching element Q1 and said second switching element Q2 beingconnected in series with a first diode D1 across said rectifier circuitDB, said first diode D1 being inserted between a high voltage side ofsaid rectifier DB and said first switching element Q1, said first diodeD1 having its cathode connected to said first switching element Q1, saidthird switching element Q3 and said fourth switching element Q4 beingconnected in series with a second diode D2 across said rectifier circuitDB, said second diode D2 being inserted between a high voltage side ofsaid rectifier DB and said third switching element Q3, said second diodeD2 having its cathode connected to said third switching element Q3, saidinductor L being connected in series with said load LD between theconnection point of said first switching element Q1 with said secondswitching element Q2 and the connection point of said third switchingelement Q3 with said fourth switching element Q4, a series circuit ofsaid first diode D1, said first switching element Q1, said inductor L,said load LD, and said fourth switching element Q4 and a series circuitof said second diode D2, said third switching element Q3, said inductorL, said load LD, and said second switching element Q2 being connected inseries with the fifth switching element Q5, said AC source, saidrectifier circuit DB, said first diode D1, said first switching elementQ1, said inductor L, said load LD, and said third diode D3 beingconnected in series across said smoothing capacitor C1, and said ACsource, said rectifier circuit DB, said second diode D2, said thirdswitching element Q3, said inductor L, said load LD, and said fourthdiode D4 being connected in series across said smoothing capacitor C1.7. The power converter as set forth in claim 1, wherein said switchingelements comprising a first switching element Q1, a second switchingelement Q2, a third switching element Q3, and a fourth switching elementQ4, each of said second switching element Q2 and said third switchingelement Q3 having a bypass allowing a reverse current to flow acrosseach switching element, said first switching element Q1 and said secondswitching element Q2 being connected in series with said inductor L andsaid load LD across said AC source, said first switching element Q1 andsaid third switching element Q3 being connected in series with saidinductor L and said load LD across said AC source, a series circuit of afirst diode D1 and a second diode D2 being connected across a seriescircuit of said second switching element Q2 and said third switchingelement Q3, a series circuit of a first smoothing capacitor C1 and asecond smoothing capacitor C2 being connected across said series circuitof said second switching element Q2 and said third switching element Q3,a diode bridge D11-D14 being inserted between the connection point ofsaid first smoothing capacitor C1 with said second smoothing capacitorC2 and said AC source, each input terminal of said diode bridge beingconnected to the connection point of said first smoothing capacitor C1with said second smoothing capacitor C2 and said AC source,respectively, said first switching element Q1 being connected betweenoutput terminals of said diode bridge D11-D14, a series circuit of athird diode D3 and a fourth diode D4 being connected across said seriescircuit of said first diode D1 and said second diode D2, said inductor Land said load LD being connected in series between the connection pointof said first diode D1 with said second diode D2 and the connectionpoint of said third diode D3 with said fourth diode D4, said fourthswitching element Q4 being connected across said series circuit of saidthird diode D3 and said fourth diode D4, a series circuit of a fifthdiode D5 and a sixth diode D6 being connected across said series circuitof said first diode D1 and said second diode D2, and said AC sourcebeing inserted between the connection point of said first diode D1 withsaid second diode D2 and the connection point of said fifth diode D5with said sixth diode D6.
 8. The power converter as set forth in claim1, wherein said switching elements comprising a first switching elementQ1, a second switching element Q2, a third switching element Q3, and afourth switching element Q4, each of said second switching element Q2and said third switching element Q3 having a bypass allowing a reversecurrent to flow across each switching element, said first switchingelement Q1 and said second switching element Q2 being connected inseries with said inductor L and said load LD across said AC source, saidfirst switching element Q1 and said third switching element Q3 beingconnected in series with said inductor L and said load LD across said ACsource, a series circuit of a first diode D1 and a second diode D2 beingconnected across a series circuit of said second switching element Q2and said third switching element Q3, a series circuit of a firstsmoothing capacitor C1 and a second smoothing capacitor C2 beingconnected across said series circuit of said second switching element Q2and said third switching element Q3, a diode bridge D11-D14 beinginserted between the connection point of said first smoothing capacitorC1 with said second smoothing capacitor C2 and one terminal of said ACsource, each input terminal of said diode bridge being connected to theconnection point of said first smoothing capacitor C1 with said secondsmoothing capacitor C2 and said terminal of said AC source,respectively, said first switching element Q1 being connected betweenoutput terminals of said diode bridge D11 D14, said one terminal of saidAC source being connected with the connection point of said first diodeD1 with said second diode D2, a diode bridge D3-D6 being insertedbetween the connection point of said first diode D1 with said seconddiode D2 and the connection point of said second switching element Q2with said switching element Q3, said diode D3 being connected in serieswith said diode D4, said diode D5 being connected in series with saiddiode D6, said inductor L and said load LD being connected in seriesbetween the connection point of said diode D3 with said diode D4 and theconnection point of said diode D5 with said diode D6, and said fourthswitching element Q4 being connected across the series circuit of saidthird diode D3 and said fourth diode D4.
 9. The power converter as setforth in claim 1, wherein said switching elements comprising a firstswitching element Q1, a second switching element Q2, a third switchingelement Q3, and a fourth switching element Q4, each of said firstswitching element Q1 and said second switching element Q2 having abypass allowing a reverse current to flow across each switching element,said first switching element Q1 and a first smoothing capacitor C1 beingconnected in series with said inductor L and said load LD across said ACsource, said second switching element Q2 and a second smoothingcapacitor C2 being connected in series with said inductor L and saidload LD across said AC source, a series circuit of said first smoothingcapacitor C1 and said second smoothing capacitor C2 being connectedacross a series circuit of said first switching element Q1 and saidsecond switching element Q2, a first diode D1 and said third switchingelement Q3 being connected in series across a series circuit of saidinductor L and said load LD, a second diode D2 and said fourth switchingelement Q4 being connected in series across said series circuit of saidinductor L and said load LD, a series circuit of said third switchingelement Q3 and said fourth switching element Q4 being connected across aseries circuit of said first diode D1 and said second diode D2, said ACsource being inserted between the connection point of said firstswitching element Q1 with said second switching element Q2 and theconnection point of said first diode D1 with said second diode D2, saidload LD, said inductor L, said AC source, and said bypass of said firstswitching element Q1 being connected in series across said firstsmoothing capacitor C1, and said bypass of said second switching elementQ2, said AC source, said inductor L, and said load LD being connected inseries across said second smoothing capacitor C2.
 10. The powerconverter as set forth in claim 1, wherein said switching elementscomprising a first switching element Q1, a second switching element Q2,a third switching element Q3, and a fourth switching element Q4, each ofsaid first switching element Q1 and said second switching element Q2having a bypass allowing a reverse current to flow across each switchingelement, said first switching element Q1 and a first diode D1 beingconnected in series with said inductor L and said load LD across said ACsource, said second switching element Q2 and a second diode D2 beingconnected in series with said inductor L and said load LD across said ACsource, a series circuit of said first diode D1 and said second diode D2being connected across a series circuit of said first switching elementQ1 and said second switching element Q2, a smoothing capacitor C1 beingconnected across said series circuit of said switching element Q1 andsaid switching element Q2, a series circuit of said third switchingelement Q3 and said fourth switching element Q4 being connected acrosssaid series circuit of said switching element Q1 and said secondswitching element Q2, said AC source being inserted between theconnection point of said first diode D1 with said second diode D2 andthe connection point of said third switching element Q3 with said fourthswitching element Q4, said inductor L and said load LD being inserted inseries between the connection point of said first switching element Q1with said second switching element Q2 and the connection point of saidthird switching element Q3 with said fourth switching element Q4, saidbypass of said second switching element Q2, said load LD, said inductorL, said AC source, and said first diode D1 being connected in seriesacross said smoothing capacitor C1, and said second diode D2, said ACsource, said inductor L, said load LD, and said bypass of said firstswitching element Q1 being connected in series across said smoothingcapacitor C1.
 11. The power converter as set forth in claim 1, whereinsaid switching elements comprising a first switching element Q1, asecond switching element Q2, a third switching element Q3, and a fourthswitching element Q4, each of said third switching element Q3 and saidfourth switching element Q4 having a bypass allowing a reverse currentto flow across each switching element, a first diode D1 and said firstswitching element Q1 being connected in series with said inductor L andsaid load LD across said AC source, said second switching element Q2 anda second diode D2 being connected in series with said inductor L andsaid load LD across said AC source, a series circuit of said switchingelement Q1 and said second switching element Q2 being connected across aseries circuit of said first diode D1 and said second diode D2, a firstsmoothing capacitor C1 and said third switching element Q3 beingconnected in series across a series circuit of said inductor L and saidload LD, said fourth switching element Q4 and a second smoothingcapacitor C2 being connected in series across said series circuit ofsaid inductor L and said load LD, a series circuit of said firstsmoothing capacitor C1 and said second smoothing capacitor C2 beingconnected across a series circuit of said third switching element Q3 andsaid fourth switching element Q4, said AC source being connected betweenthe connection point of said first diode D1 with said second diode D2and the connection point of said first smoothing capacitor C1 with saidsecond smoothing capacitor C2, said inductor L, said load LD, and saidbypass of said third switching element Q3 being connected in seriesacross said first smoothing capacitor C1, and said bypass of said fourthswitching element Q4, said load LD, and said inductor L being connectedin series across said second smoothing capacitor C2.
 12. The powerconverter as set forth in claim 1, wherein said switching elementscomprising a first switching element Q1, a second switching element Q2,a third switching element Q3, and a fourth switching element Q4, a firstdiode D1 and a first smoothing capacitor C1 being connected in serieswith said inductor L and said load LD across said AC source, a seconddiode D2 and a second smoothing capacitor C2 being connected in serieswith said inductor L and said load LD across said AC source, a seriescircuit of said first smoothing capacitor C1 and said second smoothingcapacitor C2 being connected across a series circuit of said first diodeD1 and said second diode D2, a series circuit of said first switchingelement Q1 and said second switching element Q2 being connected acrosssaid series circuit of said first diode D1 and said second diode D2, aseries circuit of a third diode D3 and said third switching element Q3being connected across a series circuit of said inductor L and said loadLD, a series circuit of a fourth diode D4 and said fourth switchingelement Q4 being connected across said series circuit of said inductor Land said load LD, a series circuit of said third switching element Q3and said fourth switching element Q4 being connected across a seriescircuit of said third diode D3 and said fourth diode D4, and said ACsource being inserted between the connection point of said first diodeD1 with said second diode D2 and the connection point of said firstswitching element Q1 with said second switching element Q2.
 13. Thepower converter as set forth in claim 1, wherein said switching elementscomprising a first switching element Q1, a second switching element Q2,a third switching element Q3, and a fourth switching element Q4, each ofsaid second switching element Q2 and said fourth switching element Q4having a bypass allowing a reverse current to flow across each switchingelement, a first diode D1, said first switching element Q1, and a seconddiode D2 being connected in series with said inductor L and said load LDacross said AC source, said first diode D1, said second switchingelement Q2, said second diode D2, and a smoothing capacitor C1 beingconnected in series with said inductor L and said load LD across said ACsource, a third diode D3, said third switching element Q3, and a fourthdiode D4 being connected in series with said inductor L and said load LDacross said AC source, said third diode D3, said fourth switchingelement Q4, said smoothing capacitor C1, and said fourth diode D4 beingconnected in series with said inductor L and said load LD across said ACsource, a series circuit of said first switching element Q1, said fourthswitching element Q4, said inductor L, and said load LD being connectedacross said smoothing capacitor C1, a series circuit of said secondswitching element Q2, said third switching element Q3, said inductor L,and said load LD being connected across said smoothing capacitor C1,said second diode D2, said AC source, said first diode D1, said load LD,said inductor L, and said bypass of said second switching element Q2being connected in series across said smoothing capacitor C1, and saidfourth diode D4, said AC source, said third diode D3, said inductor L,said load LD, and said bypass of said fourth switching element Q4 beingconnected in series across said smoothing capacitor C1.
 14. The powerconverter as set forth in claim 1, wherein said inductor being connectedwith said smoothing capacitor through a rectifying device.
 15. The powerconverter as set forth in claim 14, wherein said inductor L having aprimary winding n1 and a secondary winding n2, a current being fed tosaid load through said primary winding n1, said secondary winding n2being connected with said smoothing capacitor C1 through said rectifyingdevice, and said smoothing capacitor C1 being charged by the currentgenerated in said secondary winding.
 16. The power converter as setforth in claim 14, including: a rectifier circuit DB which rectifies theAC current from said AC source to give a DC voltage; said switchingelements comprising a first switching element Q1 and a second switchingelement Q2, said first switching element Q1 being connected in serieswith said inductor L and said load LD across said rectifier circuit DB,a first diode D1, said smoothing capacitor C1, and a second diode D2being connected in series across said inductor L, said first diode D1and said second diode D2 defining said rectifying device, and a seriescircuit of said second switching element Q2, said inductor L, and saidload LD being connected across said smoothing capacitor C1.
 17. Thepower converter as set forth in claim 15, including: a rectifier circuitDB which rectifies the AC current from said AC source to give a DCvoltage; said switching elements comprising a first switching element Q1and a second switching element Q2, said first switching element Q1 beingconnected in series with said primary winding n1 of said inductor L andsaid load LD across said rectifier circuit DB, said second switchingelement Q2, said inductor L, and said load LD being connected in seriesacross said smoothing capacitor C1, said secondary winding n2 and afirst diode D1 being connected across said smoothing capacitor C1, saidload LD and a second diode D2 being connected in series across saidprimary winding n1, and said first diode D1 defining said rectifyingdevice.
 18. The power converter as set forth in claim 14, including: arectifier circuit DB which rectifies the AC current from said AC sourceto give a DC voltage; said switching elements comprising a firstswitching element Q1 and a second switching element Q2, said firstswitching element Q1 being connected in series with said inductor L,said load LD, said smoothing capacitor C1, and said second switchingelement Q2 across said rectifier circuit DB, said first switchingelement Q1 being connected in series with said inductor L, said load LD,and a first diode D1 across said rectifier circuit DB, a series circuitof a second diode D2, said smoothing capacitor C1, and a third diode D3being inserted across said inductor L, and said second diode D2 and saidthird diode D3 defining said rectifying device.
 19. The power converteras set forth in claim 14, wherein said switching elements comprising afirst switching element Q1, a second switching element Q2, a thirdswitching element Q3, and a fourth switching element Q4, each of saidfirst switching element Q1 and said second switching element Q2 having abypass allowing a reverse current to flow across each switching element,a first diode D1, said first switching element Q1, said inductor L, andsaid load LD being connected in series across said AC source, said loadLD, said inductor L, said second switching element Q2, and a seconddiode D2 being connected in series across said AC source, said thirdswitching element Q3 and said bypass of said second switching element Q2being connected in series across a series circuit of said inductor L andsaid load LD, a third diode D3, a smoothing capacitor C1, and saidbypass of said second switching element Q2 being connected in seriesacross said inductor L, said bypass of said first switching element Q1and said fourth switching element Q4 being connected in series acrosssaid series circuit of said inductor L and said load LD, said bypass ofsaid first switching element Q1, said smoothing capacitor C1, and afourth diode D4 being connected in series across said inductor L, andsaid third diode D3 and said fourth diode D4 defining said rectifyingdevice.
 20. The power converter as set forth in claim 15, wherein saidswitching elements comprising a first switching element Q1, a secondswitching element Q2, a third switching element Q3, and a fourthswitching element Q4, each of said first switching element Q1 and saidsecond switching element Q2 having a bypass allowing a reverse currentto flow across each switching element, a first diode D1, said firstswitching element Q1, said primary winding n1 of said inductor L, andsaid load LD being connected in series across said AC source, said loadLD, said primary winding n1, said second switching element Q2, and asecond diode D2 being connected in series across said AC source, saidthird switching element Q3 and said bypass of said second switchingelement Q2 being connected in series across a series circuit of saidprimary winding n1 and said load LD, said bypass of said first switchingelement Q1 and said fourth switching element Q4 being connected inseries across said series circuit of said primary winding n1 and saidload LD, a series circuit of a third diode D3, said smoothing capacitorC1, and a fourth diode D4 being connected across said secondary windingn2, a series circuit of a fifth diode D5, said smoothing capacitor C1,and a sixth diode D6 being connected across said secondary winding n2, aseries circuit of said first switching element Q1, said primary windingn1, said load LD, said third switching element Q3 being inserted acrosssaid smoothing capacitor C1, a series circuit of said fourth switchingelement Q4, said load LD, said primary winding n1, said second switchingelement Q2 being inserted across said smoothing capacitor C1, and saidthird diode D3, said fourth diode D4, said fifth diode D5, and saidsixth diode D6 defining said rectifying device.